Fix circular dependency in workflow preventing container builds

[Copilot]

The GitHub Actions workflow had a circular dependency that prevented manual container builds from working correctly. The generate_matrix job was trying to use its own output needs.generate_matrix.outputs.file_host instead of the workflow input parameter.

This caused workflow failures when trying to rebuild specific containers like ramips-mt7621-v23.05.6, as reported in the issue where the container was missing from the registry despite the underlying ImageBuilder being available.

The fix:

Changed line 147 in .github/workflows/containers.yml from:

FILE_HOST=${{ needs.generate_matrix.outputs.file_host }}

to:

FILE_HOST=${{ github.event.inputs.file_host }}

How to rebuild missing containers:

Maintainers can now successfully rebuild missing containers by:

1. Going to the Actions tab
2. Clicking "Run workflow"
3. Setting the appropriate parameters:
   • ref: The version (e.g., v23.05.6)
   • target: The target architecture (e.g., ramips/mt7621)
4. Clicking "Run workflow"

This will build and push the container to the registry (e.g., ghcr.io/openwrt/imagebuilder:ramips-mt7621-v23.05.6).

Also included documentation (REBUILD_INSTRUCTIONS.md) explaining the rebuild process for future reference.

Fixes #182.

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