The ornamental design for a containerinsert, as shown and described.
FIG. 1 is a front, top perspective view of a container insert showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a rear, bottom perspective view thereof;
FIG. 9 is another front, top perspective view thereof, shown with environment subject matter; and,
FIG. 10 is an exploded front, top perspective view thereof.
The broken-line showing of a bowl in FIGS. 9 and 10 illustrates environmental structure. All other broken lines represent portions of the container insert. The broken lines form no part of the claimed design.