We claim the ornamental design for a stackable container, substantially as shown and described.
FIG. 1 is a top perspective view of a stackable container in accordance with the present design.
FIG. 2 is a bottom perspective view thereof.
FIG. 3 is a front elevational view thereof;
FIG. 4 is a back elevational view thereof;
FIG. 5 is a right side elevational view thereof.
FIG. 6 is a left side elevational view thereof.
FIG. 7 is a top plan view thereof.
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a perspective view of a container stack that includes the stackable container representing an environment that such container may be used in.
The broken lines in the drawing depict boundaries of the claimed design or depict environmental subject matter/unclaimed parts of the design and form no part of the claimed design.