The ornamental design for a containercover, as shown and described.
FIG. 1 is a perspective view of a container cover showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a rear elevation view thereof:
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is bottom plan view thereof.
The subject matter presented in broken lines as shown in the drawings is included for the purpose of illustrating environmental structure and forms no part of the claimed design.