build controller images for amd64 and arm64

[neoaggelos]

Summary

Also see https://github.com/canonical/cluster-api-bootstrap-provider-microk8s/pull/25

[neoaggelos]

Example build:

$ make docker-build IMG=neoaggelos/capmcpp:dev1 -j
$ make docker-manifest IMG=neoaggelos/capmcpp:dev1

# docker manifest inspect neoaggelos/capmcpp:dev1
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
   "manifests": [
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 739,
         "digest": "sha256:0b8d5f221032929d8b995619b406becccca1a114f9aa382e0dabbebabc720888",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 738,
         "digest": "sha256:33579f6cdac476c4b8796f25599d3743b93bd433b1f5a67589959f22a23b1e8a",
         "platform": {
            "architecture": "arm64",
            "os": "linux"
         }
      }
   ]
}