TRIVIAL: ADD GitHub action for Docker build (#3)

.github/workflows/docker.yml

@@ -0,0 +1,50 @@
+name: Docker release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: wandera/net-utils
+  TAG: ${{ github.ref_name }}
+
+jobs:
+  docker:
+    name: Build
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+      - name: Setup Cosign
+        uses: sigstore/cosign-installer@v3
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v2
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Setup image metadata
+        id: docker_meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+      - name: Build and push
+        uses: docker/build-push-action@v2
+        id: build
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.docker_meta.outputs.tags }}
+          labels: ${{ steps.docker_meta.outputs.labels }}
+      - name: Sign the images
+        env:
+          DIGEST: ${{ steps.build.outputs.digest }}
+        run: cosign sign --yes "${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${DIGEST}"