feat(container): add official multi-platform image

[JSChronicles]

Description

Add a batteries-included Anvil container image based on Python 3.14 slim with all supported provider dependencies installed from package metadata.

• Add an official batteries-included Anvil OCI image
• Use Python 3.14 slim with pinned base-image digests
• Install all official provider dependencies through the existing all extra
• Run as a non-root user with anvil as the entrypoint
• Support linux/amd64 and linux/arm64
• Add a reusable and manually dispatchable container workflow
• Smoke-test each platform before creating release tags
• Publish versioned images to GHCR with SBOM and provenance
• Prevent prereleases from replacing the stable latest tag
• Verify that published images are publicly accessible
• Integrate container publication with the existing release workflow
• Document local, CI, authentication, mount, and digest-pinned usage
• Add Dependabot coverage for pinned Docker base images
• Extend diagnostics for all supported provider SDKs

Checklist

• Python format, lint, and tests (ruff and pytest) were successful.
• Pre-commit hooks passed locally.
• Documentation updated if needed.
• Unit tests added or updated.