The ornamental design for a shower footrest, as shown and described.
FIG. 1 is a perspective view of shower foot rest, showing our new design;
FIG. 2 is a top view of the shower foot rest;
FIG. 3 is a bottom view of the shower foot rest;
FIG. 4 is a right side view of the shower foot rest;
FIG. 5 is a left side view of the shower foot rest;
FIG. 6 is a front view of the shower foot rest; and,
FIG. 7 is a rear view of the shower foot rest.
The broken lines depict portions of the shower foot rest that form no part of the claimed design, and environment that forms no part of the claimed design.