GHA: Add mirageos workflow (moved from toplevel)

.github/workflows/mirageos.yml

@@ -0,0 +1,41 @@
+name: Build MirageOS docker image
+
+on:
+  push:
+    paths:
+      - ".github/workflows/mirageos.yml"
+      - "docker/Dockerfile.muen-mirageos"
+
+jobs:
+  build:
+    name: Build image
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Cache Docker layers
+        uses: actions/cache@v4
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha  }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+      - name: Setup Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Log in to Container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build Docker image
+        id: docker_build
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./docker/Dockerfile.muen-mirageos
+          platforms: linux/amd64
+          push: ${{ github.ref == 'refs/heads/devel' }}
+          cache-from: type=local,src=/tmp/.buildx-cache
+          cache-to: type=local,dest=/tmp/.buildx-cache
+          tags: ghcr.io/codelabs-ch/muen-mirageos:latest