Move agentk build to CNG

Rather than doing Sign agentk images (#629 - closed) and Remove 32 bit arm CPU support for agentk (#567 - closed), we can just move to agentk image build to CNG that will do that for us.

➜  ~ docker manifest inspect registry.gitlab.com/gitlab-org/build/cng/gitlab-kas:17-5-stable 
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
   "manifests": [
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3043,
         "digest": "sha256:59e3ecaddf860f8560ba62c6e5d5e3872cf4c32333bff4977634ab66da92c8fc",
         "platform": {
            "architecture": "arm64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
         "size": 3043,
         "digest": "sha256:b76de3b3aca3ed460b6962af4686c451e8a39d2b0d88ecca95dfd2ae60324875",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      }
   ]
}
➜  ~ docker manifest inspect registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:v17.5.0
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.oci.image.index.v1+json",
   "manifests": [
      {
         "mediaType": "application/vnd.oci.image.manifest.v1+json",
         "size": 2555,
         "digest": "sha256:8ae519b6d85323f6b9bd4c5a59e6a22255447bf8c658a5ad255a59e0f09298e5",
         "platform": {
            "architecture": "amd64",
            "os": "linux"
         }
      },
      {
         "mediaType": "application/vnd.oci.image.manifest.v1+json",
         "size": 2555,
         "digest": "sha256:9a0dba48a2174c5ecc6d22900c2406a164d4dbac1800fc38b1a2b9667e0c9c6d",
         "platform": {
            "architecture": "arm",
            "os": "linux",
            "variant": "v7"
         }
      },
      {
         "mediaType": "application/vnd.oci.image.manifest.v1+json",
         "size": 2555,
         "digest": "sha256:8a7e5bd342dec14879d642dd21dae5394790fdbdeb5f7464fef79657e3eb3376",
         "platform": {
            "architecture": "arm64",
            "os": "linux",
            "variant": "v8"
         }
      }
   ]
}

Release notes

We are moving the agentk container registry from its project specific location to the Cloud Native GitLab registry. If you mirror the agentk container to a local registry, you should change your mirror source to the CNG registry.

If you use the official Helm-charts, they will start deploying from the new location seamlessly.