Fix Docker CI smoke test by loading Buildx output and installing package non-editably

[Copilot]

Pull request template

Description

The Build Docker image GitHub Actions job failed at smoke-test time because chen:ci was not loaded into the local Docker daemon, and the container runtime then surfaced a package import failure path. This PR makes the Docker CI build output runnable in-job and ensures the runtime image has an importable chen installation.

• CI workflow fix
  • Enable local image loading in Buildx so subsequent docker run chen:ci ... uses the just-built image.
• Docker packaging fix
  • Replace editable install in the builder stage with a standard install so the runtime image resolves chen correctly.
• Minimal diff
  • One workflow flag + one Dockerfile install-mode change; no unrelated behavior changes.

# .github/workflows/ci.yml
- uses: docker/build-push-action@v5
  with:
    push: false
    load: true
    tags: chen:ci

# docker/Dockerfile
RUN pip install --no-cache-dir --upgrade pip \
    && pip install --no-cache-dir ".[server,dev]"

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement
• Benchmark / KPI change

Related issues

How was this tested?

• make test-fast passes locally
• make lint passes
• make typecheck passes (or N/A)
• New tests added for new functionality
• MockBackend-based tests added (no GPU required)

Checklist

• My code follows the style guidelines (ruff, line length 100)
• I have run make format on my code
• I have added type hints to all new public APIs
• I have updated the CHANGELOG / README if needed
• My changes generate no new warnings
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with make test-fast

Notes for reviewers

Focus review on the two-line behavioral changes in:

• .github/workflows/ci.yml (load: true under Docker Buildx step)
• docker/Dockerfile (non-editable install in builder stage)

Both changes are directly tied to making the CI Docker smoke-test path executable.