devcontainer workflow enhancement

[huxuan]

• dev container should not be pushed for merge request in GitLab CI/CD.

    - if [[ "$CI_PIPELINE_SOURCE" != "merge_request_event" ]]; then
        docker buildx build . \
          --build-arg PYTHON_VERSION=${PYTHON_VERSION} \
          --cache-to type=registry,ref=${CI_REGISTRY_IMAGE}/dev-cache:py${PYTHON_VERSION},mode=max \
          --file .devcontainer/Dockerfile \
          --push \
          --tag ${CI_REGISTRY_IMAGE}/dev:py${PYTHON_VERSION} \
          --target dev;
      fi

• Resolve warning in Dockerfile.

#4 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 9)
#4 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 70)
#4 WARN: FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 84)

• prod container is not built and tested in GitLab CI/CD.
• Rethink whether devcontainer workflow should be triggered when mounted files in prod container changes.

[huxuan]

devcontainer workflow is mostly for building and testing the new dev containers. Since the ci workflow is supposed to have the same environment as dev containers. Though this is true only for GitLab but not for GitHub currently, but that is a different story. In this way, we do not need to build a same dev container and test it again.