The ornamental design for an indicator for a container, as shown and described.
FIG. 1 is a front elevation view of a first embodiment of an indicator for a container showing our new design;
FIG. 2 is left side view thereof, where the right side view being identical thereto;
FIG. 3 is a top plan view thereof, where the bottom plan view being identical thereto;
FIG. 4 is a rear view thereof; and,
FIG. 5 is a front view of an alternate configuration of FIG. 1, wherein the indicator has been exposed to electromagnetic radiation changing its appearance.
The broken lines shown in the drawings illustrate portions of the indicator for a container that form no part of the claimed design.