CI: Add release workflow.

.github/workflows/release.yml

@@ -0,0 +1,30 @@
+name: Deploy versioned & latest
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  deploy-base:
+    name: Deploy base image
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Get the version
+        id: get_version
+        run: echo ::set-output name=VERSION::${GITHUB_REF/refs\/tags\//}
+        shell: bash
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build base image
+        run: docker build -t ghcr.io/mu-editor/mu-appimage:latest .
+      - name: Tag same base image with the versioned tag
+        run: docker tag ghcr.io/mu-editor/mu-appimage:latest ghcr.io/mu-editor/mu-appimage:${{ steps.get_version.outputs.VERSION }}
+      - name: Push base images to ghcr.io
+        run: |
+          docker push ghcr.io/mu-editor/mu-appimage:${{ steps.get_version.outputs.VERSION }}
+          docker push ghcr.io/mu-editor/mu-appimage:latest