feat: add docker-build composite action and build template

.gitea/workflows/build-template.yml

@@ -0,0 +1,26 @@
+# Example build.yml for a libreshop component repo.
+# Copy to .gitea/workflows/build.yml and remove the "on:" override below.
+#
+# Replace the existing multi-step build.yml with these ~20 lines.
+
+name: build
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: git.librete.ch/libretech/runner-image:v1
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: libreshop/actions/.gitea/actions/docker-build@main
+        with:
+          registry: ${{ secrets.REGISTRY }}
+          registry_user: ${{ secrets.REGISTRY_USER }}
+          registry_pass: ${{ secrets.REGISTRY_PASS }}
+          publish: ${{ secrets.PUBLISH_ENABLED }}