Build Microsoft container image embedding database

[Tian1996]

/claim #22

Summary

• Add a helper that fetches official Microsoft/devcontainers image metadata from devcontainers/images.
• Store image metadata, searchable text, and optional embedding JSON in a local SQLite database.
• Add a repeatable CLI script for building data/msft_container_images.db.
• Document the optional MSFT_CONTAINER_IMAGES_DB_PATH setting.

Tests

• python3 -m unittest tests/test_msft_container_images.py
• python3 -m py_compile helpers/msft_container_images.py scripts/build_msft_container_image_embeddings.py tests/test_msft_container_images.py
• git diff --check
• python3 scripts/build_msft_container_image_embeddings.py --skip-embeddings --db-path /tmp/msft_container_images_test.db stored 18 image records