ci: build multi arch docker images using buildx

ignore docker login in pull requests

.github/workflows/docker.yml

@@ -2,8 +2,9 @@ name: Docker CI
 
 on:
   push:
-    # runs on every commit
-    branches: ["master"]
+    branches:
+      - master
+  pull_request:
 
 permissions:
   contents: read
@@ -15,11 +16,16 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - name: Build the docker image
-        run: docker build -t steelscheme/steel:latest .
-
       - name: Authenticate to the Docker Hub
-        run: docker login -u steelscheme -p ${{ secrets.DOCKER_HUB_PWD }}
+        if: ${{ github.event_name != 'pull_request' }}
+        uses: docker/login-action@v3
+        with:
+          username: steelscheme
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Setup Docker Buildx
+        uses: docker/setup-buildx-action@v3
 
-      - name: Push the docker image
-        run: docker push steelscheme/steel:latest
+      - name: Build docker image
+        run: |
+          docker buildx build --push --platform linux/amd64,linux/arm64 -t steelscheme/steel:latest .