ci: Build Docker for `arm64` arch

- Trivial upgrades:
  - docker/build-push-action@v5 => v6

.github/workflows/build-docker.yaml

@@ -14,6 +14,12 @@ on:
       ref:
         required: true
         type: string
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: The ref to build from.
+        required: true
+        type: string
 
 # By default, run all commands in a bash shell.  On Windows, the default would
 # otherwise be powershell.
@@ -32,6 +38,11 @@ jobs:
           ref: ${{ inputs.ref }}
           submodules: recursive
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          platforms: linux/amd64,linux/arm64
+
       - name: Build
         shell: bash
-        run: docker buildx build .
+        run: docker buildx build --platform linux/amd64,linux/arm64 .

.github/workflows/publish-docker.yaml

@@ -49,6 +49,11 @@ jobs:
           submodules: recursive
           fetch-tags: true
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          platforms: linux/amd64,linux/arm64
+
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
@@ -56,24 +61,26 @@ jobs:
           password: ${{ secrets.DOCKERHUB_CI_TOKEN }}
 
       - name: Push to Docker Hub
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           # Important: use actions/checkout source, which has the right tags!
           # Without context: ., this action will try to fetch git source
           # itself, and it will be unable to determine the correct version
           # number.
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ secrets.DOCKERHUB_PACKAGE_NAME }}:${{ inputs.tag }}
 
       - name: Push to Docker Hub as "latest"
         if: ${{ inputs.latest }}
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           # Important: use actions/checkout source, which has the right tags!
           # Without context: ., this action will try to fetch git source
           # itself, and it will be unable to determine the correct version
           # number.
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ secrets.DOCKERHUB_PACKAGE_NAME }}:latest