Write registry auth without docker CLI

.gitea/workflows/build.yaml

@@ -15,6 +15,11 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          cache-binary: false
+
       - name: Determine image name and tags
         id: meta
         env:
@@ -46,11 +51,17 @@ jobs:
 
       - name: Log in to Gitea registry
         if: steps.meta.outputs.is_release == 'true'
-        uses: docker/login-action@v3
+        env:
-        with:
+          # The job image (node:20-bullseye) has no docker CLI, so docker/login-action
-          registry: ${{ vars.REGISTRY }}
+          # can't run. buildx reads ~/.docker/config.json directly, so write the auth
-          username: ${{ secrets.REGISTRY_USER }}
+          # there ourselves. Secrets via env keep them out of the templated script.
-          password: ${{ secrets.REGISTRY_PASSWORD }}
+          REGISTRY: ${{ vars.REGISTRY }}
+          REGISTRY_USER: ${{ secrets.REGISTRY_USER }}
+          REGISTRY_PASSWORD: ${{ secrets.REGISTRY_PASSWORD }}
+        run: |
+          mkdir -p "$HOME/.docker"
+          AUTH="$(printf '%s:%s' "$REGISTRY_USER" "$REGISTRY_PASSWORD" | base64 -w0)"
+          printf '{"auths":{"%s":{"auth":"%s"}}}' "$REGISTRY" "$AUTH" > "$HOME/.docker/config.json"
 
       - name: Build and push release image
         if: steps.meta.outputs.is_release == 'true'