Use cleaned destination path for indexing image volumes

We use filepath.Clean() to remove trailing slashes to ensure that
when we supercede image mounts with mounts from --volume and
--mount, paths are consistent when we compare. Unfortunately,
while we used the cleaned path for the destination in the mount,
it was accidentally not used to index the maps that we use to
identify what to supercede, so our comparisons might be thrown
off by trailing slashes and similar.

Fixes #5219

Signed-off-by: Matthew Heon <matthew.heon@pm.me>
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