The ornamental design for a cover for a container, as shown and described.
FIG. 1 is a front perspective view of a cover for a container showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;
FIG. 5 is a rear elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line showing of the body of a container in FIG. 1 is included for the purpose of illustrating environment and forms no part of the claimed design.