Build multiarch images

I was unable to get Duo Workflow running on my M1 MacBook. I use lima directly (not the colima wrapper), but the problem seems to be that the image only released for amd64 while my docker host is arm64. I don't know how it works for others with colima, but I think we should release multiarch images anyway.

To verify that it is indeed a multiarch image after this change, you can run

docker manifest inspect registry.gitlab.com/gitlab-org/duo-workflow/default-docker-image/workflow-generic-image:d3da7fcd

For me, the output is:

{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
   "manifests": [
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 1581,
         "digest": "sha256:dcb566e1ccfc9ed92c146eb2acc007c151ac68407700bc6d8b615a41e0322002",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 1581,
         "digest": "sha256:de6c745647fc788c80f456a9755247efc24feae145c65321ba512112888d0ead",
         "platform": {
            "architecture": "arm64",
            "os": "linux"
         }
      }
   ]
}