add action to automatically update garm runner

.gitea/workflows/garm-image.yml

@@ -0,0 +1,66 @@
+name: Build garm image
+
+on:
+  schedule:
+    - cron: "13 3 * * *"
+  push:
+    branches:
+      - main
+    paths:
+      - .gitea/workflows/garm-image.yml
+      - apps/garm/image-source.env
+      - docker/garm/**
+  workflow_dispatch:
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Load pin data
+        shell: bash
+        run: |
+          set -euo pipefail
+          source apps/garm/image-source.env
+          echo "GARM_COMMIT=${GARM_COMMIT}" >> "$GITHUB_ENV"
+          echo "GARM_COMMIT_NUMBER=${GARM_COMMIT_NUMBER}" >> "$GITHUB_ENV"
+          echo "GARM_IMAGE=${GARM_IMAGE}" >> "$GITHUB_ENV"
+
+      - name: Verify commit number
+        shell: bash
+        run: |
+          set -euo pipefail
+          tmpdir="$(mktemp -d)"
+          trap 'rm -rf "$tmpdir"' EXIT
+          git clone --filter=blob:none https://github.com/cloudbase/garm.git "$tmpdir"
+          expected="$(git -C "$tmpdir" rev-list --count "$GARM_COMMIT")"
+          if [ "$expected" != "$GARM_COMMIT_NUMBER" ]; then
+            echo "Pin mismatch: expected r${expected}, got r${GARM_COMMIT_NUMBER}" >&2
+            exit 1
+          fi
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to gitea registry
+        uses: docker/login-action@v3
+        with:
+          registry: gitea.lumpiasty.xyz
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: docker/garm/Dockerfile
+          push: true
+          build-args: |
+            GARM_COMMIT=${{ env.GARM_COMMIT }}
+          tags: |
+            ${{ env.GARM_IMAGE }}
+          labels: |
+            org.opencontainers.image.source=https://github.com/cloudbase/garm
+            org.opencontainers.image.revision=${{ env.GARM_COMMIT }}