`docker pull` returns `denied: access forbidden` from private gitlab registry

If this is an authenticated registry, then you need to run docker login <registryurl> on the machine where you are building this.

This only needs to be done once per host. The command then caches the auth in a file

$ cat ~/.docker/config.json
{
    "auths": {
        "https://index.docker.io/v1/": {
            "auth": "......="
        }
    }
}