Publish multi-arch images using `docker manifest`

What does this Merge Request do?

It changes the public-facing agentk image tags to multi-arch manifests, supporting AMD64 and ARM64 architectures to start with.

WARNING

The -arm64 and -amd64 tags are NOT PUBLIC API.

They are a temporary solution until bazel's rules_docker supports it natively, see https://github.com/bazelbuild/rules_docker/issues/1599 for tracking.

Manual testing

I've verified that the manifest generated in https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/jobs/2080743626 is working on both amd64 (my laptop) and arm64 (a short lived EC2 instance). The post-merge manifests also look good, for example:

$ docker manifest inspect registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:stable
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
   "manifests": [
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 591,
         "digest": "sha256:bb5d06428457a49e0139044a0985c8ecf14d3df1caf13b307f6469fd0b156b19",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 591,
         "digest": "sha256:820463e1421c10d155351d69c0f89dab6ab374df8a5f4d9fcb1f4f7bae64b7d8",
         "platform": {
            "architecture": "arm64",
            "os": "linux"
         }
      }
   ]
}

Related issues

Resolves #213 (closed)