Add documentation for rebuilding missing containers

Co-authored-by: aparcar <16000931+aparcar@users.noreply.github.com>

REBUILD_INSTRUCTIONS.md

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+# Instructions for Rebuilding Missing Containers
+
+This document explains how to rebuild missing containers like the reported `ramips-mt7621-v23.05.6` container.
+
+## The Issue
+
+The container `ghcr.io/openwrt/imagebuilder:ramips-mt7621-v23.05.6` was missing due to a workflow issue that prevented manual container builds from working correctly.
+
+## The Fix
+
+Fixed a circular dependency in `.github/workflows/containers.yml` where the `generate_matrix` job was trying to use its own output for `file_host` instead of the workflow input.
+
+## How to Rebuild a Missing Container
+
+To rebuild the missing `ramips-mt7621-v23.05.6` container:
+
+1. Go to the [Actions tab](https://github.com/openwrt/docker/actions/workflows/containers.yml) 
+2. Click "Run workflow"
+3. Fill in the parameters:
+   - **ref**: `v23.05.6`
+   - **target**: `ramips/mt7621` 
+   - **file_host**: (leave empty for default)
+   - **prefix**: (leave empty for default)
+4. Click "Run workflow"
+
+This will build and push the container to `ghcr.io/openwrt/imagebuilder:ramips-mt7621-v23.05.6`.
+
+## General Pattern
+
+For any missing container `TARGET-VERSION`:
+- **ref**: The version (e.g., `v23.05.6`, `main`)  
+- **target**: The target architecture (e.g., `ramips/mt7621`, `x86/64`)
+
+The resulting container will be tagged as:
+- `ghcr.io/openwrt/imagebuilder:TARGET-VERSION`
+- `ghcr.io/openwrt/imagebuilder:TARGET-REF` (e.g., `ramips-mt7621-v23.05.6`)
+
+## Testing
+
+After the workflow completes successfully, you can test the container:
+
+```bash
+podman run -it ghcr.io/openwrt/imagebuilder:ramips-mt7621-v23.05.6
+```
+
+Inside the container, run:
+```bash
+./setup.sh
+make image PROFILE=generic
+```