Split ENV ROCM_HOME and ENV PATH into separate commands to fix variable
expansion issue. When ROCM_HOME and PATH were in the same ENV line,
${ROCM_HOME} expanded to empty string since it wasn't defined yet.
This was causing 'ray: command not found' in init containers.
- Use BuildKit syntax 1.7 with cache mounts for apt/uv
- Switch from pip to uv for 10-100x faster installs (ADR-0014)
- Add OCI Image Spec labels for container metadata
- Add HEALTHCHECK directives for orchestration
- Add .dockerignore to reduce context size
- Update Makefile with buildx and lint target
- Add retry logic to ray-entrypoint.sh
Refs: ADR-0012 (uv), ADR-0014 (Docker best practices)
