[ci] Fix uv cache path in extralit-server Dockerfile

[JonnyTran]

Problem

The server Dockerfile mounts the uv build cache at /root/.cache/uv while the runtime container runs as the non-root extralit user. The cache directory is unreadable by that user, causing redundant downloads on rebuilds and possible permission errors during package operations.

Files

• extralit-server/docker/server/Dockerfile (around the RUN --mount=type=cache,target=/root/.cache/uv directive and the USER extralit line)

Fix

• Change the mount target to /home/extralit/.cache/uv, or
• Define a build-time ENV UV_CACHE_DIR=... under a path the runtime user owns and mount there.

Acceptance criteria

• Build succeeds with the new mount path.
• docker run --rm <image> ls -la $UV_CACHE_DIR is readable by extralit.