The ornamental design for a storagecontainer with corner supports, as shown and described.
FIG. 1 is a perspective view of a storage container showing a new design;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a side view thereof; and,
FIG. 5 is a side view of a storage container showing my new design in an in-use configuration.
The evenly-spaced broken lines shown in the drawings are directed to environment, are for informational purposes only, and form no part of the claimed design.