Debug image-build without checkout

.forgejo/workflows/image-build.yaml

@@ -1,10 +1,6 @@
 name: Image Build Probe
 
 on:
-  push:
-    paths:
-      - ".forgejo/workflows/image-build.yaml"
-      - "Dockerfile"
   workflow_dispatch:
 
 env:
@@ -16,24 +12,20 @@ jobs:
   image-build:
     runs-on: container-build
     steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-
-      - name: Build and push image
+      - name: Build and push image (no checkout)
         env:
           REGISTRY_USER: ${{ secrets.REGISTRY_USER }}
           REGISTRY_TOKEN: ${{ secrets.REGISTRY_TOKEN }}
         run: |
-          set -eu
-          mkdir -p "${HOME}/bin"
+          set -eux
+          mkdir -p "${HOME}/bin" buildctx
           wget -qO- https://download.docker.com/linux/static/stable/x86_64/docker-27.3.1.tgz             | tar xz --strip-components=1 -C "${HOME}/bin" docker/docker
           export PATH="${HOME}/bin:${PATH}"
           docker version
           echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" -u "${REGISTRY_USER}" --password-stdin
-          TAG="${GITHUB_SHA:-manual}"
-          SHORT="${TAG:0:7}"
+          echo forgejo-image-build-probe-ok > buildctx/probe.txt
+          printf 'FROM alpine:3.20\nCOPY probe.txt /\n' > buildctx/Dockerfile
           IMAGE="${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}"
+          docker build -t "${IMAGE}:dispatch-test" buildctx
+          docker push "${IMAGE}:dispatch-test"
+          echo "pushed ${IMAGE}:dispatch-test"