The ornamental design for a collapsiblecontainer, as shown and described.
FIG. 1 is a perspective view of a collapsible container showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top view thereof.
The top portion and bottom portion of the collapsible container is shown in broken lines for illustrative purposes only and forms no part of the claimed design.