Switch Docker image to Alpine multi-stage build

[fsistemas]

Moves the official Docker image to an Alpine base with a multi-stage build, producing a smaller image (~99 MB) that continues to run as a non-root user.

The PostgreSQL driver is compiled from source in the build stage (no musl wheel), and only the runtime venv plus libpq ship in the final image.

Verified locally:

• amd64 and arm64 multi-arch build and run
• PostgreSQL, MySQL, and SQLite queries (named, connection-scoped, and parametrized)
• JSON / CSV / Excel output
• Runs as non-root (uid=1000)

Docs: RELEASING.md notes the source-compile of the driver (slower arm64 leg under emulation) and the cached-arch example is updated.