The ornamental design for a convertiblecontainer, as shown and described.
FIG. 1 is a front elevational view of a convertible container showing my new design;
FIG. 2 is a left-side view thereof;
FIG. 3 is a right-side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The evenly-dashed broken lines in the drawings depict the portions of the design that form no part of the claim. The dot-dot-dash broken lines in the drawings define the bounds of the claim and form no part thereof.