.github/workflows: Build the Arch Linux image for pull requests

... when there are changes in the 'images/arch' directory or in the
GitHub workflow itself.

https://github.com/containers/toolbox/pull/1308

.github/workflows/arch-images-pr.yaml

@@ -0,0 +1,31 @@
+name: Build the arch-toolbox image for PRs
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - images/arch/**
+      - .github/workflows/arch-images-pr.yaml
+
+jobs:
+  build-and-push-images:
+    name: Build the arch-toolbox image for PRs
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Build the arch-toolbox image
+        uses: docker/build-push-action@v3
+        with:
+          context: images/arch
+          file: images/arch/Containerfile
+          platforms: linux/amd64
+          push: false
+          no-cache: true
+          tags: quay.io/toolbx/arch-toolbox:latest