-FIELD OF THE INVENTION- The present invention relates to an improved trailer, especially for transportation of containers and for uploading/unloading of goods contained in said containers. -BACKGROUND OF THE INVENTION- The transportation of various goods and many kinds of materials, especially on long distance travels, since approximately last fifty years has advantageously made by using containers, in particular in order to avoid the uploading and unloading of the goods at every change of transport means (trucks, trains, ships and airplanes). Moreover, if stowed into containers since the departure, the goods reaches final destination with integrity and in sealed conditions without the necessity of packing them for protecting from any intermediate manipulations. Basically, a container is a large metallic case or box, particularly sturdy and with standardized dimensions, having coupling means for the anchorage with fixing means of a loading platform of a truck. The transport on road of the containers comes usually carried out with articulated truck-trailer whose tractor can be uncoupled from the trailer when this last one is engaged from the operations of upload¬ ing/unloading of the container. At present, trucks adapted to road-transportation of containers can be provided with fixed trailer, or in alternative with dumping o semi-dumping trailer provided with tilting frame for realizing the discharge by sliding of the container out from the back side of the truck. The fixed trailers are mostly used for the transport of palletized freight for unloading/uploading it with cranes, maintaining the container steady fixed over the trailer, while the trailers provided with tilting frame is used in case of container filled with loose materials such as, for example cereals, clay, metals or other similar loose materials. In case of loose materials, currently the uploading of the container is realized by using small apparatus like escavator or similar, for instance with the so called moving-earth machines. Such way to operate has the remarkable drawback of being slow and also more complicated. Moreover, such operating methodology is also much expensive. The aim of the present invention is to provide an improved trailer for overcoming the above-mentioned drawbacks. In particular, it is an object of the present invention to provide an im¬ proved trailer using of which facilitates in quick way the upload¬ ing/unloading of goods contained in containers during their transporta¬ tion. -SUMMARY OF THE INVENTION- According to the invention, is realized an improved trailer for containers transportation, especially for uploading/unloading of goods contained inside a container, comprising supporting means for supporting bottom surface of said container; characterized in that at least a pair of said supporting means are pivotable mounted on said trailer; and pushing means are provided for pushing up said bottom surface of said con¬ tainer and enabling dumping of said container around a rotation axe defined by said pair of said supporting means. -BRIEF DESCRIPTION OF THE DRAWINGS- - Fig. 1 shows a lateral view of a first preferred but non exclusive em¬ bodiment of improved trailer of the present invention, supporting a container; - Fig. 2 shows a lateral view of a detail of the trailer as shown in Fig.1 in a first operative position; - Fig. 3 shows a lateral view of a detail of the trailer as shown in Fig.1 in a second operative position; - Fig. 4 shows a rear side of the trailer as shown in Fig.1 ; - Fig. 5 and Fig. 6 show in exploded perspective views details of trailer as shown in Fig.1 ; - Fig. 7 is a partly schematic and sectional view of details of trailer as shown in Fig.4 to Fig. 6; - Fig. 8 and Fig. 9 show in lateral views two different operative positions of trailer as shown in Fig.1 ; - Fig. 10 shows a lateral view of a second preferred embodiment of improved trailer of the present invention, supporting a container; - Fig. 11 shows a lateral view of a detail of the trailer as shown in Fig.10 in a first operative position; - Fig. 12 shows a lateral view of a detail of the trailer as shown in Fig.1 in a second operative position; - Fig. 13 shows a rear side of the trailer as shown in Fig.10; - Fig. 14 and Fig. 15 show in exploded perspective views details of trailer as shown in Fig.10; - Fig. 16 is a partly schematic and sectional view of details trailer as shown in Fig.13 to Fig. 15; - Fig. 17 and Fig. 18 show in lateral views two different operative posi¬ tions of trailer as shown in Fig.10; - Fig. 19 shows, in a rear side view, a third preferred embodiment of improved trailer of the present invention, supporting a container; - Fig. 20 and Fig. 21 show in exploded perspective views details of trailer as shown in Fig.19; and - Fig. 22 is a partly schematic and sectional view of details of trailer as shown in Fig.19 to Fig. 21. -DESCRIPTION OF THE PREFERRED EMBODIMENTS- By the examination of the annexes drawings, it is described the trailer of the present invention equipped of support elements on which to rest and to berth, with means of coupling, the bottom surface of a container normally predisposed in the four corners of said container. The aforesaid supports are pivotable mounted, coaxially in pair, around two perpendicular horizontal axes in respect of the movement direction of a truck along a road, especially one for the pair of front supports and the other for the pair of back supports. In that way, the front side or the back side of the container, pushed up by at least one hydraulic hoister (for example an hydraulic cylinder or piston or equivalent), can dump forward or backward to permit uploading or unloading of loose materials placed into the container by rotating around the common axe of one of the pair of abovecited support element, while the other pair is temporar¬ ily disengaged from trailer. Specifically, in a first preferred embodiment of present invention shown in Fig.1 to Fig. 9, a container Q is placed resting onto the improved trailer R, according the invention, especially on four support elements 1 , each comprising a cantilever bracket 1 A coming out from back side of a pin 1 B (see fig.5 and fig. 6); said pin 1 B is adapted to roto-coupling with an element 2 having a hollow cylinder 2A and a portion 2B. As shown in Fig. 7, hollow cylinder 2A is adapted to support said pin 1 B at each end, and rests in semicircular pocket 6A of longitudinal frame member 6 of trailer R, which puts on two pair of transversal appendage members 2C of said hollow cylinder 2A, in way that to permit the inser¬ tion of pins 5, provided with well-known (not shown) fixing means for avoiding unthread (see Fig. 4). The portion 2B is protruding from medium zone of cylinder 2A, for resting against the bottom surface of container Q, at least with lower area 2D of element 2 in way that to receive the upper end of one of the hydraulic hoisters 3,4 supported parallel to rotation axes of support elements 1. In that manner, the hydraulic hoisters 3,4 are able to realize the dump¬ ing of the container Q around rotation axe of one pair of said support elements 1. A transversal hole 1 C on pin 1 B and transversal hole 2E on cylinder 2A are adapted to realize the insertion of a stop member, especially for avoiding unthread of support element 1 when container Q is not located on trailer R. During operation of improved trailer R according to its first embodiment, the dumping of container Q, on forward as shown in Fig.8 or on back¬ ward as shown in Fig. 9, is simply and rapidly realized by unthreading, respectively, the hydraulic hoister 3 or 4, after removal of a couple of pins 5 fixed to longitudinal frame member 6, and through the lifting of cylinder 2A pushed high by the same hoister 3 o 4. In a second embodiment of the trailer R, as shown in Fig. 10 to Fig. 18, each support element 10 of four support elements 10, on which the container Q is fixed, comprises a cantilever bracket 1OA coming out from back side of a pin 10B (see fig. 14 and fig. 15); said pin 10B is adapted to roto-coupling with an a hollow cylinder 13A of two hollow cylinders 13A of an element 13; said hollow cylinders 13A are con¬ nected each other by a portion 13C of element 13 (see fig. 13 and fig .16). The portion 13C is adapted to rest against the bottom surface of con¬ tainer Q at least with lower area 13D of element 13 in way that to receive the upper end of an hydraulic hoister 14 supported parallel to rotation axes of support elements 10. In that manner, the hydraulic hoister 14 is able to realize the dumping of the container Q around rotation axe of one of said cylinder 13A and of the pair of pins 10B supported by said cylinder 13A on its ends. Infact, each of said hollow cylinders 13A is revolving supported by cylindrical seat of two element 12 (see fig. 11 and fig. 12) fixed to longitudinal frame members 16, especially in semicircular pockets 16A through appendage members 12A which put the same longitudinal frame members 16, in way that to permit the insertion of pins 15 pro¬ vided with well-known (not shown) fixing means for avoiding unthread (see Fig. 10 and Fig. 11). The correct positioning of element 13 in respect of longitudinal frame members 16 is realized by the fact that each of four element 12 is located between a fixed collar 13B (see Fig. 13 to Fig. 16) of said cylinder 13A and a sleeve 11 , which is inserted onto cylinder 13A together with element 12 and fixed by a transversal pin on hole 11 A, on hole 13E of cylinder 13A and on hole 10C of support element 10, so to avoid accidental unthreading, especially in absence of container Q. Therefore, during operation of improved trailer R according to its second embodiment, the dumping of container Q, on forward as shown in Fig.17 or on backward as shown in Fig. 18, is simply and rapidly real- ized by unthreading the unique hydraulic hoister 14, after removal of the pins 15 fixed to longitudinal frame members 16. In that way, container Q and element 13 are able to rotate around pair of elements 12 fixed to longitudinal frame members 16. According to a third embodiment of improved trailer R of the present invention, as shown in fig. 19 to fig. 22, a pin 2OB protruding from a cantilever bracket 2OA of each of four support element 20; the bottom surface of container Q therefore rests on said cantilever bracket 2OA. The pin 2OB, before insertion within an hollow cylinder 22A of an ele¬ ment 22, passes through a cylindrical seat of a revolving element 21 which is lined up to cylinder 22A and fixed to one of two longitudinal frame members 24 of trailer R by pair of appendage members 21 A (see Fig. 20 and Fig. 21) which put said longitudinal frame member 24 and provided with holes passed by a pin 23 (see Fig. 19). Said two hollow cylinders 22A of element 22 have medium areas rigidly fixed each other by a portion 22B adapted to rest against bottom sur¬ face of container Q at least with its central part located in seat 22D so to receive upper end of an hydraulic hoister 25 (in mixed line in Fig. 19). In that way, said cylinders 22A are adapted to be pushed by said hoister 25 so realizing the dumping of the container Q together with element 22 and support element 20, in respect of a pair of element 21 fixed by pins 23 to longitudinal frame members 24. Transversal hole 2OC on pin 2OB of support 20 and holes 22C on hollow cylinder 22A allow an insertion of stopping means. Therefore, during operation of improved trailer R according to its third embodiment, the dumping of container Q, on forward or on backward, is simply and quickly realized by unthreading the unique hydraulic hoister 25, after removal of the pins 23 fixed to longitudinal frame members 24. In such way, container Q and support elements 20 are able to rotate around the axes of the elements 21 fixed to longitudinal frame members 24. Although the present invention has been described with reference to the preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.