ci: Set up emulation and build for arm64

Previous commits enable this to work.

Emulation is necessary to build arm64 on an amd64 host machine [1].

[1]: https://github.com/docker/build-push-action/blob/965c6a410d446a30e95d35052c67d6eded60dad6/docs/advanced/multi-platform.md

.github/workflows/ci.yml

@@ -22,9 +22,11 @@ jobs:
       # and hyphens.
       run: echo "TAG=branch-${GITHUB_REF_NAME//[^A-Za-z0-9._-]/-}" | tee -a $GITHUB_ENV
 
+    - uses: docker/setup-qemu-action@v2
+
     - run: ./devel/start-localhost-registry $REGISTRY_PORT
 
-    - run: ./devel/build-to-localhost -p linux/amd64 -r localhost:$REGISTRY_PORT -t $TAG
+    - run: ./devel/build-to-localhost -p linux/amd64,linux/arm64 -r localhost:$REGISTRY_PORT -t $TAG
 
     - if: github.event_name != 'pull_request'
       uses: docker/login-action@v1