feat(docker): make container user configurable via CF_UID/CF_GID (#118)

* feat(docker): make container user configurable via CF_UID/CF_GID

Add support for running compose-farm containers as a non-root user
to preserve file ownership on mounted volumes. This prevents files
like compose-farm-state.yaml and web UI config edits from being
owned by root on NFS mounts.

Set CF_UID, CF_GID, and CF_HOME environment variables to run as
your user. Defaults to root (0:0) for backwards compatibility.

* docs: document non-root user configuration for Docker

- Add CF_UID/CF_GID/CF_HOME documentation to README and getting-started
- Add XDG config volume mount for backup/log persistence across restarts
- Update SSH volume examples to use CF_HOME variable

* fix(docker): allow non-root user access and add USER env for SSH

- Add `chmod 755 /root` to Dockerfile so non-root users can access
  the installed tool at /root/.local/share/uv/tools/compose-farm
- Add USER environment variable to docker-compose.yml for SSH to work
  when running as non-root (UID not in /etc/passwd)
- Update docs to include CF_USER in the setup instructions
- Support building from local source with SETUPTOOLS_SCM_PRETEND_VERSION

* fix(docker): revert local build changes, keep only chmod 755 /root

Remove the local source build logic that was added during testing.
The only required change is `chmod 755 /root` to allow non-root users
to access the installed tool.

* docs: add .envrc.example for direnv users

* docs: mention direnv option in README and getting-started

Dockerfile

@@ -16,5 +16,9 @@ RUN apk add --no-cache openssh-client
 COPY --from=builder /root/.local/share/uv/tools/compose-farm /root/.local/share/uv/tools/compose-farm
 COPY --from=builder /usr/local/bin/cf /usr/local/bin/compose-farm /usr/local/bin/
 
+# Allow non-root users to access the installed tool
+# (required when running with user: "${CF_UID:-0}:${CF_GID:-0}")
+RUN chmod 755 /root
+
 ENTRYPOINT ["cf"]
 CMD ["--help"]