Add image-build workflow for T04 CI proof

.forgejo/workflows/image-build.yaml

@@ -0,0 +1,37 @@
+name: Image Build Probe
+
+on:
+  push:
+    paths:
+      - ".forgejo/workflows/image-build.yaml"
+      - "Dockerfile"
+  workflow_dispatch:
+
+env:
+  REGISTRY: forgejo.coulomb.social
+  IMAGE_NAME: coulomb/forgejo-actions-probe
+
+jobs:
+  image-build:
+    runs-on: container-build
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Login to Forgejo registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push
+        run: |
+          set -eu
+          TAG="${GITHUB_SHA:-manual}"
+          SHORT="${TAG:0:7}"
+          IMAGE="${REGISTRY}/${IMAGE_NAME}"
+          docker build -t "${IMAGE}:latest" -t "${IMAGE}:${SHORT}" .
+          docker push "${IMAGE}:latest"
+          docker push "${IMAGE}:${SHORT}"
+          echo "pushed ${IMAGE}:latest and ${IMAGE}:${SHORT}"