Manually exec'ing `restic tag` command results in mismatch of user execution

Steps to reproduce:

1. Start container
2. Make sure `restic` repository has snapshots
3. Check `restic` snapshot cache ownership: `docker exec ls -lR /tmp/.cache/restic | grep root`
4. Notice nothing is owned by `root`
5. Modify `restic` snapshot tags: `docker exec restic tag --add vanilla `
6. Check `restic` snapshot cache ownership again: `docker exec ls -lR /tmp/.cache/restic | grep root`
7. Notice that the cache's modified snapshot(s) are now owned by `root`

I haven't tested with other `restic` "writable" functions but I suppose they'll probably do the same.

Start by reproducing the listed docker exec commands and inspecting how the container runs manually invoked restic operations and writes /tmp/.cache/restic. Done means writable restic commands preserve the expected container user ownership in the cache; the payload names no source files or tests to inspect.

Written by the indexing model from the issue text.