Align Docker release workflow with the ebooks project's registry setup

Switch from a dynamically-derived registry/GITHUB_TOKEN login to the
same pattern already proven in the ebooks project: a fixed
git.arnef.de/arnef/nidus registry/image, docker/login-action with
REGISTRY_USERNAME/REGISTRY_PASSWORD repo secrets, docker/metadata-action
for tag/label generation (version, major.minor, latest, short SHA), and
a final buildx/system prune cleanup step. Also add a manual
workflow_dispatch trigger.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/docker-release.yml

@@ -1,64 +1,58 @@
-name: Docker release
-
-# Builds and publishes a multi-arch (linux/amd64 + linux/arm64) Docker
-# image whenever a version tag (e.g. "v1.2.3") is pushed, or a release is
-# published for one. Written against the GitHub Actions syntax, which
-# Gitea Actions runs unmodified — the ${{ github.* }} context and
-# secrets.GITHUB_TOKEN are provided as compatibility aliases by Gitea's
-# act runner, so this workflow works as-is on either platform.
-#
-# The image is pushed to this server's own container registry, addressed
-# as "<host>/<owner>/<repo>" (e.g. git.example.com/owner/nidus on Gitea,
-# or ghcr.io/owner/nidus if this repo is ever mirrored to GitHub).
+name: Docker Image bauen und veröffentlichen
 
 on:
   push:
-    tags:
-      - "v*"
+    tags: ["v*"]
   release:
     types: [published]
+  workflow_dispatch: {}
+
+env:
+  REGISTRY: git.arnef.de
+  IMAGE_NAME: arnef/nidus
 
 jobs:
   docker:
     runs-on: ubuntu-latest
-    permissions:
-      contents: read
-      packages: write
     steps:
-      - name: Check out repository
+      - name: Repository auschecken
         uses: actions/checkout@v4
 
-      - name: Determine registry, image name, and tags
-        id: vars
-        run: |
-          registry="$(echo "${{ github.server_url }}" | sed -E 's#^https?://##')"
-          image_name="$(echo "${{ github.repository }}" | tr '[:upper:]' '[:lower:]')"
-          image="${registry}/${image_name}"
-          version="${GITHUB_REF_NAME#v}"
-
-          {
-            echo "registry=${registry}"
-            echo "image=${image}"
-            echo "tags=${image}:${version},${image}:latest"
-          } >> "$GITHUB_OUTPUT"
-
-      - name: Set up QEMU
+      - name: QEMU einrichten (für Cross-Platform-Builds)
         uses: docker/setup-qemu-action@v3
 
-      - name: Set up Docker Buildx
+      - name: Docker Buildx einrichten
         uses: docker/setup-buildx-action@v3
 
-      - name: Log in to the container registry
+      - name: Bei Registry anmelden
         uses: docker/login-action@v3
         with:
-          registry: ${{ steps.vars.outputs.registry }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
-      - name: Build and push
+      - name: Metadaten (Tags/Labels) ermitteln
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha,prefix=,format=short
+
+      - name: Image bauen und pushen (amd64 + arm64)
         uses: docker/build-push-action@v6
         with:
           context: .
           platforms: linux/amd64,linux/arm64
           push: true
-          tags: ${{ steps.vars.outputs.tags }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+
+      - name: Build-Umgebung aufräumen
+        if: always()
+        run: |
+          docker buildx prune --all --force
+          docker system prune --all --force --volumes