Publish a multi-platform Docker image

Addresses https://github.com/realm/SwiftLint/issues/4531

.github/workflows/docker.yml

@@ -4,43 +4,42 @@ on:
   push:
     branches:
       - main
+      - jp/multi-platform-docker
     tags:
       - '*'
 
 jobs:
   build:
     runs-on: ubuntu-20.04
-  
     steps:
     - uses: actions/checkout@v3
-
     - name: Extract DOCKER_TAG using tag name
       if: startsWith(github.ref, 'refs/tags/')
       run: |
         echo "DOCKER_TAG=${GITHUB_REF/refs\/tags\//}" >> $GITHUB_ENV
-    
     - name: Use default DOCKER_TAG
       if: startsWith(github.ref, 'refs/tags/') != true
       run: |
-        echo "DOCKER_TAG=latest" >> $GITHUB_ENV
-    
+        echo "DOCKER_TAG=preview" >> $GITHUB_ENV
     - name: Set lowercase repository name
       run: |
         echo "REPOSITORY_LC=${REPOSITORY,,}" >>${GITHUB_ENV}
       env:
         REPOSITORY: '${{ github.repository }}'
-      
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v2
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v1
-      
-    - name: Login to GitHub registry
-      uses: docker/login-action@v1 
+      with:
+        platforms: linux/amd64,linux/arm64
+    - name: Log in to GitHub registry
+      uses: docker/login-action@v1
       with:
         username: ${{ github.actor }}
         password:  ${{ secrets.GITHUB_TOKEN }}
         registry: ghcr.io
-    
     - uses: docker/build-push-action@v2
       with:
+        platforms: linux/amd64,linux/arm64
         push: true
         tags: ghcr.io/${{ env.REPOSITORY_LC }}:${{ env.DOCKER_TAG }}