Registry.gitlab.com not working in Docker Proxy of Nexus repository

I’m trying to configure Nexus with a Docker Proxy for registry.gitlab.com to proxy the gitlab-runner-helper image (registry.gitlab.com/gitlab-org/gitlab-runner/gitlab-runner-helper:x86_64-dcfb4b66) - but an error appears in the Nexus log:

anonymous org.sonatype.nexus.repository.docker.internal.auth.BearerScheme - Failed to retrieve docker bearer token
2023-05-25 15:25:00,394+0200 WARN  [qtp702342186-465944]  anonymous org.apache.http.impl.auth.HttpAuthenticator - BEARER [complete=true] authentication error: Could not retrieve token from https://gitlab.com/jwt/auth. Status code: 403

I can pull the image without a proxy, but this one seems to fail. What am I missing?

My cross post from the Nexus forum: Docker Proxy for registry.gitlab.com not working - Sonatype Nexus Repository - Sonatype Community