Fix docker container crashing when run with non-1000 user

[na-ji]

Summary

• UV_CACHE_DIR was hardcoded to /home/mediamanager/.cache/uv, owned by uid 1000. Running the container as any other non-root uid (e.g. user: "1001:1001" in docker-compose) caused uv to crash on startup with a permission error, since it couldn't create the cache dir.
• Switched UV_CACHE_DIR to /tmp/uv-cache in both Dockerfile and metadata_relay/Dockerfile. /tmp is world-writable via the sticky bit, so this works for any uid/gid without needing ownership changes.

Fixes #566

Test plan

• Built the dependencies stage from the pre-fix Dockerfile and reproduced the exact error from the issue running as uid 1001 and uid 99.
• Built the dependencies stage from the fixed Dockerfile and confirmed uv run works as uid 1001, uid 99, uid 1000 (default), and root.

Summary by CodeRabbit

• Chores
  • Updated the application’s container build configuration to use a temporary location for package-manager caching.
  • Preserved cache ownership settings to maintain consistent build behaviour.

[coderabbitai]

📝 Walkthrough

Walkthrough

The Dockerfiles move the uv cache from the mediamanager home directory to /tmp/uv-cache. The main Dockerfile retains UID/GID 1000 ownership for its BuildKit cache mount. Other build settings remain unchanged.

Changes

Container cache configuration

Layer / File(s)	Summary
Relocate uv cache paths Dockerfile, metadata_relay/Dockerfile	Both Dockerfiles use /tmp/uv-cache for the uv cache and BuildKit cache mount. The main Dockerfile retains UID/GID 1000 ownership settings. Other build steps remain unchanged.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Poem

A rabbit hops through /tmp with care,
Finding fresh cache space there.
The builds stay neat, the paths align,
With fewer home-directory binds.
Squeak, the containers start on time!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #566 by moving the uv cache to a location usable by arbitrary non-root UIDs and GIDs.
Out of Scope Changes check	✅ Passed	Both Dockerfile changes directly support the linked issue and PR objective; no unrelated changes are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the Docker container fix and is directly related to the changes, although it exceeds the ideal 50-character length.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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