Add docker cross-compile for arm64

This makes use of the docker buildx to cross-compile images for amd64/arm64. It is worth noting that there are ongoing issues with the Go compiler and qemu (used by buildx/buildkit) and the solution I ended up using here was to limit the affinity to `go build`. Better solutions may be forthcoming.

Refs:
https://github.com/golang/go/issues/24656
https://bugs.launchpad.net/qemu/+bug/1696773

This relates to issue #11 regarding container images built for running on Raspberry Pi.

example.yaml

@@ -22,7 +22,7 @@ spec:
         - --config=/config/config.yaml
         - --content-server=http://localhost:9090
         - --agree-eula
-        image: docker.io/criticalstack/quake:v1.0.2
+        image: docker.io/criticalstack/quake:v1.0.3
         name: server
         ports:
         - containerPort: 8080
@@ -40,7 +40,7 @@ spec:
         - q3
         - content
         - --seed-content-url=http://content.quakejs.com
-        image: docker.io/criticalstack/quake:v1.0.2
+        image: docker.io/criticalstack/quake:v1.0.3
         name: content-server
         ports:
         - containerPort: 9090