Currently, getNestedMountpoints sorts using sort.Strings, which would
sort the following strings in this exact order:
/dir/nested, /dir/nested-vol, /dir/nested.vol, /dir/nested/double, /dir/nested2
Because of this, "nested/double" is returned as well, even though it shouldn't
have been. This issue is worse on Windows, where the path separator is typically
the backslash.
This commit addresses this issue by checking if a nested mount point has been
previously seen or not.
This patch cleans up pkg/util/mount/* and pkg/util/volume/* to always
use filepath.Join instead of path.Join. filepath.Join is preferred
because path.Join can have issues on Windows.
Since the runtime may try to create mount points within
the sandbox, it will fail if the mount point is within
a read-only API data volume, like a secret or configMap
volume.
Create any needed mount points during volume setup.
