Examples to push to multi-registries (#154) and cache to registry

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

README.md

@@ -34,6 +34,8 @@ ___
   * [Isolated builders](#isolated-builders)
   * [Multi-platform image](#multi-platform-image)
 * [Advanced usage](#advanced-usage)
+  * [Push to multi-registries](#push-to-multi-registries)
+  * [Cache to registry](#push-to-multi-registries)
   * [Local registry](#local-registry)
   * [Leverage GitHub cache](#leverage-github-cache)
   * [Complete workflow](#complete-workflow)
@@ -235,6 +237,102 @@ jobs:
 
 ## Advanced usage
 
+### Push to multi-registries
+
+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.
+
+<details>
+  <summary><b>Show workflow</b></summary>
+
+  ```yaml
+  name: ci
+  
+  on:
+    push:
+      branches: master
+  
+  jobs:
+    multi-registries:
+      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: .
+            file: ./Dockerfile
+            platforms: linux/386,linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64,linux/ppc64le,linux/s390x
+            push: true
+            tags: |
+              user/app:latest
+              user/app:1.0.0
+              ghcr.io/user/app:latest
+              ghcr.io/user/app:1.0.0
+  ```
+</details>
+
+### Cache to registry
+
+You can import/export cache from a cache manifest or (special) image configuration on the registry.
+
+<details>
+  <summary><b>Show workflow</b></summary>
+
+  ```yaml
+  name: ci
+
+  on:
+    push:
+      branches: master
+
+  jobs:
+    registry-cache:
+      runs-on: ubuntu-latest
+      steps:
+        -
+          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: Build and push
+          uses: docker/build-push-action@v2
+          with:
+            push: true
+            tags: user/app:latest
+            cache-from: type=registry,ref=user/app:latest
+            cache-to: type=inline
+  ```
+</details>
+
 ### Local registry
 
 For testing purposes you may need to create a [local registry](https://hub.docker.com/_/registry) to push images into: