The ornamental design for a stackablecontainer, as shown and described.
FIG. 1 is a front elevation view of a stackable container with a lid thereon showing my new design, the right side elevation view, left side elevation view and rear elevation view being identical thereto;
FIG. 2 is a front elevation view of the stackable container with the lid removed, the right side elevation view, left side elevation view and rear elevation view being identical thereto;
FIG. 3 is a side view of the stackable container;
FIG. 4 is a top view of the stackable container;
FIG. 5 is a side view of the lid;
FIG. 6 is a bottom plan view of the lid;
FIG. 7 is a top plan view of the stackable container with the lid thereon; and,
FIG. 8 is a cross sectional view of the stackable container taken along line 8—8 of FIG. 7.