Technical Field The present invention relates to a locking system to be used in a container comprising walls and a base, wherein at least a section of at least one wall is movable attached to the container. The locking system comprises at least one locking device having at least one lock lever arm rotatable about at least one pivot axis and at least one latch interacting with at least one lock recess, which lock lever arm is yieldable biased by a spring into engagement with the lock recess for releasably attaching the wall section to the container. Description of Prior Art Containers using collapsible and/or detachable side walls and/or pivotable sections of the walls, i.e. doors or hatches giving access to the interior of the container without having to open or detach a whole side wall are well known, in particular containers for transporting and storing various items, such as piece goods and bulk material. The side walls of empty collapsible containers are for example folded into a position in which the container height is considerably lower taking up much less space when transported to be filled again, or one or more walls or broken wall sections are detached for replacement. Moreover, one or more walls or wall sections may also be opened so that the inside of the container is accessible by movement/pivoting the walls or wall sections either inwards or outwards of the container. The container wall sections are often pivotally attached at their lower ends to the container, i.e. the base or deck of the container so that they can be folded from an upright or vertical position to an essentially horizontal flat position or other suitable angle in relation to its upright position for creating a sufficiently big opening in the container giving access to its interior. To ensure that the walls, e.g. side walls in prior art containers are securely held in their upright position they often have locking/catching and holding mechanisms that are manually operated by using biased handles, often one pair of handles being attached at a first side wall and another pair of handles being attached at a second side wall opposite the first one. The locks are often moved in a translation movement against the biasing force by manually gripping the handles by the finger tips or the thumb and pulling them strongly when the side walls are locked together. One end of each handle with a latch engaging a lock recess in the adjacent side wall is then moved away from this side wall out of engagement with the lock recess and releases the side walls from each other for folding and/or detaching. WO 03/055753 discloses a collapsible container with a deck, four collapsible side walls, and a lock mechanism in the form of a biased pivoting lock lever securing the side walls in the upright position. The lever is biased by a powerful spring acting on the pivoting point of the lock lever. Summary of the Invention The main object of the present invention is to provide a container with a locking system having at least one handle that is easily reachable and accessible from both the inside and the outside of the container, minimizes the effort for an operator counteracting the biasing force in the handle when actuating the handle for moving one or more walls or wall sections in relation to the container, and is ergonomically shaped and located at a more ergonomical position on the container. These objects are achieved by a locking system to be used in a container comprising walls and a base, wherein at least a section of at least one wall is movable attached to the container. The locking system comprises at least one locking device in the form of a handle having at least one lock lever arm rotatable about at least one pivot axis and at least one latch interacting with at least one lock recess, which lock lever arm is yieldable biased by a spring into engagement with the lock recess for releasably attaching the wall section to the container. The spring is adapted to engage the lock lever arm at a distance from the lock lever arm pivot. In one embodiment of the locking system according to the invention the lock lever arm is a double-armed lever with a first spring engaging arm and a second arm carrying or forming the latch, and in other embodiments of the locking system the lock lever arm is a single- lever arm in which the spring is adapted to act on the lock lever arm between its two ends, preferably at its middle section. The spring acting distance from the lock lever arm pivot is at least 10% to 100% of the lock lever arm length, preferably at least 50% of the lock lever arm length. The spring in yet another embodiment is a leaf spring. The locking device in another embodiment is arranged on the movable wall section and the lock recess is arranged on the adjoining wall section or, in yet another embodiment, the lock recess is arranged on the movable wall section and the locking device is arranged on the adjoining wall section. In a further embodiment the spring is attached to and adapted to engage the free end of the first arm, and in another embodiment the spring is adapted to act on the lock lever arm at its middle section. In a preferred embodiment of the locking system, the lock lever arm, i.e. the handle, has a U-shaped cross- section similar to a U-bar for straddling at least a part of an edge of the wall section. In a second preferred embodiment, the lock lever arm is countersunk in a recess at the wall section edge. By the improved handle of the invention to be used for pivoting/moving and/or detaching at least a section of the walls of a container and locking the same in their upright position, several advantages are obtained. By arranging the spring so that it engages the lock lever arm at a distance from the lock lever arm pivot the required spring force is decreased compared to prior art locking mechanisms and the strain or force on an operator actuating the handle is decreased. This is especially the case when the operator uses his/hers thumbs as an anvil or dolly abutting the container and the other fingers grip the handle for working against the spring force and only the fingers and associated muscles are used. In the preferred embodiment, the outside of the hands of the operator face upwards so that their inside, i.e. the inner surface of the wrists/hands of the operator are facing downwards and abut against the upper side or surface of the handles inside the wall section edge recess with the fingers inside the container, whereby the angle for the hands in relation to the fore arms are more ergonomical and the operator can use the weight of the body for actuating the handles and decrease the effort for the operator and the strain on hands, wrists and arms even more. This also means that the thumb and the other fingers are mainly relieved from load in that the operator works against a rigid support, i.e. the container with a main part of the inside of the hand without explicitly using the muscles in the hand. Brief Description of the Drawings The present invention will now be described in more detail with reference to the enclosed drawings, in which: Fig 1 is a perspective view of a container with handles according to a first embodiment of the invention, Fig 2 is a enlarged cut-out view of one of the handles in fig 1, Fig 3 is a perspective view of a container with handles according to a second embodiment of the invention, Fig 4 is an enlarged cut-out view of a corner area of the container with one of the handles in fig 3, Fig 5 is an enlarged cut-out view of a corner area of a container with a handle similar to the handles in fig 3, Fig 6 is a view from above of the handle in fig 5, Fig 7 is a lateral view partly in section along line A-A showing the handle in fig 6 in its locking position, Fig 8 is an enlarged cut-out view of a corner area of a container with a handle according to a third embodiment of the invention, Fig 9 is an enlarged cut-out view of the handle in fig 8, and Fig 10 is an enlarged cut-out view from above of the handle in fig 9. Detailed Description of the Invention Figs 1-10 illustrate a container 20 comprising walls 30, 40, a deck 50, and locking systems 10 having at least one locking device comprising a biased handle 10 according to the invention for releasably locking the whole walls and/or sections of the walls of the container together. Moreover, one or more covers/lids (not shown) may also be releasably and/or detachably attached by means of the locking system 10 to the container, e.g. by fitting them into mating grooves on the upper edges 33, 43 of the container or pivotally attaching them by hinges adjoining at least one edge of the cover with one container edge. The handles 10 can be placed on the detachable cover or on the container 20. The container also comprises at least one supporting part, in this case, skids 80, even though these skids are not necessary, as is readily understood by a skilled person, each skid having a width W. The container is described as having a square shape with four side walls, two first side walls 30 and two second side walls 40, and four locking systems 10, but may of course only have three triangular shaped side walls with a triangular shaped deck 50 forming a closed pyramid or more than four walls with for example a pentagonal, hexagonal or octagonal shaped deck resulting in a corresponding container shape. As shown in fig 3, the container walls 30, 40 may also be equipped with one or more wall sections 34, 44, in this case, two wall sections, i.e. two doors or hatches 34, 44, compared to the container in fig 1 without hatches, and/or one or more locking systems 10. The hatches are movable inwards or outwards of the container and movable in relation to the walls, whereby the walls 30, 40 are moved in relation to the container or its base 50. The movable wall sections 34, 44 can be a part of the associated wall 30, 40 or the whole wall and can be opened to give access to the interior of the container and/or be detached and attached separately or together with the walls. The handles 10 in figs 1-10 lock the four side walls 30, 40 together in the upright/vertical position but can of course also be arranged on the hatches 34, 44 as is schematically shown with broken/dashed lines in fig 3. The walls can be inclined in relation to each other, e.g. as a pyramid. The handle 10 comprises at least one pivoting lock lever 11, 12 being rotatably attached at a first end 11a, 12a to a wall 30, 40 by means of a pivot 13 at an upper end 32, 42 of each wall and means 60 in the form of a spring for biasing the lever. Each of the walls 30, 40 is pivotally attached at a lower end 31, 41 to the deck 50 but may of course be pivotally attached together at their upper edges 32, 42 or their side edges, if desired. The locking lever 11, 12 has a second end lib, 12b adapted to engage the adjoining second wall 30, 40 with a latch or tip 14 fitting into a locking recess 15 on the adjacent wall for simultaneously holding this wall . As shown in figs 2 and 4-7, the latch 14 is an integrated part of the lock lever 11, 12. In figs 9 and 10, the latch 14 is a separate element that is separately movable and actuated by the first end 11a biasing it towards the lock recess 15 by means of a second spring 70 integrated in the latch 14. This separate latch 14 may of course also be integrated in the lock lever arm. The handles 10 are shown attached in pairs to two opposite first side walls 30 but may instead be attached to the other two second side walls 40 on the inside or outside of the container 20 or in a combination with one inner pair and one outer pair of handles or even two inner pairs. The handles do not have to be arranged in pairs, instead, only one handle per side wall 30 or 40 can be used by placing a first handle 10 on a first side wall 30, a second handle 10 diagonally across on the other opposite first side wall 30, a third handle on a second side wall 40 at the opposite corner of the first handle, and a fourth handle diagonally across from the third handle on the opposite other second side wall 40. The handles 10 may also be connected by linkage or wire systems (not shown) so that when one handle in each pair is actuated, i.e. pulled or pushed by an operator the other handle in the pair of handles follows the movement of the first handle "passively" by means of the linkages or wires pulling or pushing the second handle. The pair- wise arrangement of the handles 10 on each side wall 30 or 40 is preferred, and it is most preferred to place the handles on the short sides of the container 20. The walls 30, 40 or the wall sections 34, 44 can also be pivotally attached to the container 20 and each other in the vertical direction at their adjoining corner areas, i.e. their side edges, instead of the shown horizontal direction. The walls may also be slidably attached without the possibility of rotational/pivotal movement by sliding one pair of two opposite walls with their edges into grooves at the adjoining edges of the other pair of walls and/or the deck 50 and then locking the walls together with a suitable number of locking systems 10. A first embodiment of the handle 10 according to the invention is shown in figs 1 and 2. In fig 1 one pair of handles 10 are placed on the outside of the first side wall 30 and the other pair of handles is placed on the inside of the opposite side wall 30 facing the inside of the container 20. By employing one outer pair of handles and one inner pair of handles opposite the first pair in the container 20 an operator folding the side walls 30, 40 can lower the side wall with the outer handles 10 first and then step inside the container 20 if a large container is used or just reach across the container deck 50 if a smaller container is used for actuating the inner handles on the opposite side wall, so that the operator does not have to walk around the container to the other opposite side. In fig 1 the handles 10 are placed at the upper end 32 of the first container side wall 30 or the upper end 42 of the second container side wall 40 on a distance seen in the vertical direction from an upper edge 33 or 43 of the first or second side wall 30, 40 and from the lower side wall ends 31, 41 such that each handle is easily reachable and actuated with no extra effort by a thumb when placing the inside of the fingers or the inside of the hand, i.e. the palm, on the upper side wall edge 33, 43. This first embodiment of the handle in fig 2 is a double-armed lever with a first arm 11 and a second arm 12 extending essentially perpendicularly in relation to each other and interconnected at the pivot 13 that extends in a direction being essentially perpendicular to the plane of the associated side wall 30, 40. In the position of the side walls 30, 40 locked together as shown in figs 1 and 2 the first lock lever arm 11 extends essentially horizontally and the second lock lever arm 12 extends essentially vertically. Each arm 11, 12 has its first end 11a, 12a rotatably attached to the pivot 13 by means of snap fitting but may be releasebly attached to the pivot by any other suitable means. The second lever arm 12 has the catch-like latch 14 at its second end 12b for engaging a lock recess 15 in the second side wall 40. The biasing means 60 is provided on the free second end lib of the first lock lever arm 11 and acts thereon at a distance from the pivot 13 being about 100% of the lock lever arm length. Fig 2 shows the biasing spring 60 in the form of a leaf or blade spring attached at the top surface or face of the handle 10, i.e. the first lock lever arm 11, with one end 60a attached to the tip of the free second end lib of the first lock lever arm 11 with the other free end 60b extending towards the pivot 13, this free spring end can also extend in the opposite direction, i.e. away from the pivot. The free spring end 60b acts upwards against a support 16 on the side wall 30 and presses the latch 14 into the latch recess or opening 15 in the side wall 40 when locking it against the first side wall 30. By attaching the spring 60 at this point of the lever 11 it acts on the handle 10 on a distance as far as possible from the pivot 13 and the axis of rotation, whereby the spring force can be as low as possible due to its longest possible lever length. This is considered in all three embodiments of the handle 10 and minimizes the required handle releasing force from the operator and also optimizes the dimensions of the handle because the shorter spring acting length the stronger or sturdier handle 10 and spring 60. In figs 3-7 a second embodiment of the handle 10 according to the invention is shown. The handle is countersunk in a recess 35 at the upper end 32, 42 of the side walls 30, 40 with a form and a length L essentially corresponding to the handle contour and somewhat larger so that a clearance around the handle is achieved for enabling sufficient movement/displacement of the handle. In fig 4 the handle 10 is arranged with its upper surface 10a somewhat below the upper surface of the side wall edge 33, 43 so that if another second container 20 (not shown) is put or stacked on top of the first container and has any dirt or the like protruding downwards from its deck 50 this protrusion will not accidentally press on the handle 10 from above releasing the side walls 30, 40. Furthermore, the width W of the skid parts 80 on one container 20 being stacked on top of another container is always larger than the length L of each edge recess 35 so that the skids of the top container never can come in contact with the handle 10 and press it downwards, accidentally releasing the walls 30, 40 or wall sections 34, 44 from each other. In fig 5 the upper surface 10a of the handle 10 is shown essentially flush with the side wall edge 33, 43, whereby the container 20 with this second type of handle in fig 5 is not intended for stacking. Fig 6 shows the second type of handle 10 from above and fig 7 shows a lateral view of this handle partly in section along line A-A with the latch 14 and the pivot 13 attached to the first wall 30 at the right with the second wall 40 to the left. The spring 60 is attached at the underside of the handle at the first end 11a of the lever 11 with the free second lever end lib extending away from the pivot 13 and acting downwards on the side wall 30. At the same time, the spring 60 presses/acts upwards against a support 16 on the lock lever arm 11, in this embodiment, a downwardly extending rim, so that the latching tip 14 and the handle 10 are forced upwards. The support rim 16 is located at a distance from the pivot 13 and the axis of rotation for the lock lever arm 11 being at least 50% of the lock lever arm length, and provides a sufficient snap-in force for the handle when it automatically engages the wall sections 30, 34, 40, 44. The handle 10 in figs 4, 5, and 6 covers a part of the upper edge 33, 43 of the associated side wall 30, 40 bilaterally, i.e. the handle has a U-shaped cross-section and the legs of the U with the corresponding surfaces forming a U-bar extending downwards over the two sides of the wall, i.e. the U-bar straddles at least a part of the upper edge 33, 43 of the associated wall 30, 34, 40, 44. Figs 8-10 show a third embodiment of a handle 10 according to the invention placed in a recess 35 essentially flush with or somewhat countersunk in the surface of the wall 30, 40. In this embodiment, the pivot axis 13 extends in a direction being essentially in parallel with the plane of the wall section 30, 34, 40, 44, and the latch 14 is a separately movable element biased by the lock lever arm 11 by means of the second spring 70 into engagement with the lock recess 15. The handle 10 in figs 8-10 is actuated in a way similar to a "push-button" by pushing the lock lever arm 11 towards the wall 30 or 40 on the container 20 in the direction shown with an arrow in fig 8 and against the spring force from the first spring 60 acting on the lock lever arm 11 and the support 16. Then, the first end 11a pushing the latch 14 in a direction being essentially perpendicular to the pivot axis 13 for engaging the lock recess 15 is moved to the left in fig 10 and releases the latch or decreases the biasing force at least until the force from the operator pushing the wall 30, 40 overcomes the spring force and the latch comes loose from the lock recess 15. In some cases, the whole lock lever arm 11 acts as a spring together with the two springs 60, 70 for enhancing the actuation of the handle 10. Other positions for the locking handles 10 according to the invention are shown schematically pairwise with broken/dashed lines on the upper edges of the two hatches 34, 44 in fig 3. The straddling handle according to the second embodiment may also be attached to the container without any pivot 13 and instead be slidable fitted with its edges in grooves in the associated wall 30, 34, 40, 44 and recess 35 defining only a translation movement for the handle so that when the handle is actuated a separately connected latch 14 is pulled or pushed in or out of the lock recess 15. These handles 10 according to the invention do not have to be biased by springs 60, 70 because they can be moved between a locking position into a release position and back manually but a biased handle is preferred. The pivot 13 for each handle 10 according to the invention may also be plased in any suitable position within the confinement/dimension/extension of the handle or even outside its extension. The handles may also be placed relatively far from a separate latch 14 and actuate it by means of wires or linkage systems or an integrated latch can be relatively long for reaching into the lock recess 15. The wall sections 34, 44 may also be pivotally attached with their upper edges adjacent or even on the upper edges 33, 43 of the walls 30, 40 instead of being attached with hinges at a distance from the upper wall edges 33, 43. One wall 30 may also be pivotally attached at its upper edge 32 to the upper edge 42 of adjoining fixed walls. In figs 3 and 4, the handle 10 is placed at distance from the adjoining walls 40 with a part of the wall 30 separating the handle and the adjoining wall 40, and in fig 5 the handle is placed close to the adjoining wall. The container 20 may have the movable cover (not shown) with the lock recess 15 arranged on the container and the locking device 10, 11, 12, 13, 14, 60 arranged on the cover for releasably attaching it to the container. Alternatively, the locking device 10, 11, 12, 13, 14, 60 may be arranged on the container and the lock recess 15 may be arranged on the cover.