Add optional external checkout inputs to docker-build-publish workflow

.github/workflows/docker-build-publish.yml

@@ -10,6 +10,35 @@ on:
         required: true
         type: string
 
+      checkout_repo:
+        description: >
+          Optional external repo to checkout (owner/name). If set, repo is checked out
+          into checkout_path.
+        required: false
+        type: string
+        default: ""
+
+      checkout_ref:
+        description: >
+          Optional git ref (branch/tag/SHA) for checkout_repo. Defaults to repo default branch.
+        required: false
+        type: string
+        default: ""
+
+      checkout_path:
+        description: >
+          Path to checkout checkout_repo into.
+        required: false
+        type: string
+        default: "external-src"
+
+      checkout_fetch_depth:
+        description: >
+          Fetch depth for checkout_repo (0 = full history).
+        required: false
+        type: string
+        default: "0"
+
       registry_host:
         required: true
         type: string
@@ -53,6 +82,17 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Checkout external repository
+        if: ${{ inputs.checkout_repo != '' }}
+        uses: actions/checkout@v4
+        with:
+          repository: ${{ inputs.checkout_repo }}
+          ref: ${{ inputs.checkout_ref }}
+          server-url: ${{ github.server_url }}
+          token: ${{ secrets.ci_token }}
+          path: ${{ inputs.checkout_path }}
+          fetch-depth: ${{ inputs.checkout_fetch_depth }}
+
       - name: Load env vars
         if: ${{ inputs.env != '' }}
         run: |