The ornamental design for a pistoncontainer, as shown and described.
FIG. 1 is a perspective view of a piston container showing my new design;
FIG. 2 is a front elevational view thereof, the rear being a mirror image;
FIG. 3 is a right side elevational view thereof, the left side being a mirror image;
FIG. 4 is a top plan view thereof; the broken lines in FIG. 4 are included for the purpose of illustrating a portion of the piston container that forms no part of the claimed design; and,