The ornamental design for a maskcontainer, as shown and described.
FIG. 1 is a front perspective view of a mask container shown in open position;
FIG. 2 is a perspective view of the mask container shown with an example mask;
FIG. 3 is a side perspective view of the mask container;
FIG. 4 is a rear perspective view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a left-side view, the right-side view being a mirror image thereof;
FIG. 7 is a back view thereof;
FIG. 8 is a top view thereof; and,
FIG. 9 is a bottom view thereof.
The broken lines illustrating a mask represent environment, and the broken lines illustrating a clip represent portions of the article that form no part of the claimed design.