Add publish step

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

.github/workflows/publish.yaml

@@ -0,0 +1,31 @@
+name: publish
+
+on:
+  push:
+    tags:
+      - '*'
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+        with:
+          fetch-depth: 1
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          registry: ghcr.io
+      - name: Local build
+        id: local_build
+        uses: docker/build-push-action@v2
+        with:
+          outputs: "type=registry,push=true"
+          platforms: linux/amd64,linux/arm/v6,linux/arm64
+          tags: ghcr.io/alexellis/registry-creds:${{ github.sha }}