Enhance documentation

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/advanced/push-multi-registries.md

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+# Push to multi-registries
+
+* [Docker Hub and GHCR](#docker-hub-and-ghcr)
+
+## Docker Hub and GHCR
+
+The following workflow will connect you to [DockerHub](https://github.com/docker/login-action#dockerhub)
+and [GitHub Container Registry](https://github.com/docker/login-action#github-container-registry) and push the
+image to these registries.
+
+```yaml
+name: ci
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
+      -
+        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.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: Login to GitHub Container Registry
+        uses: docker/login-action@v1 
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.CR_PAT }}
+      -
+        name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: |
+            user/app:latest
+            user/app:1.0.0
+            ghcr.io/user/app:latest
+            ghcr.io/user/app:1.0.0
+```