Dependency Proxy/Docker - Invalid Reference Format Error

Hi All,

I am trying to use GitLab as a dependency proxy for docker containers. I am new to GitLab and I have installed GitLab server and runner (docker executor) in AWS EC2 instances. The server is running on RHEL 8 and runner is on Amazon Linux 2. The GitLab version is 14.10.1 and docker version in the runner is 20.10.13. HTTP proxy environment variables have been configured in http-proxy.conf for both runner and docker.

I am able to pull from docker registry using image: alpine:latest in .gitlab-ci.yml in my project. However, using ${CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX}/alpine:latest results in the below error:
Pulling docker image …
WARNING: Failed to pull image with policy “always”: invalid reference format (manager.go:203:0s)
ERROR: Job failed: failed to pull image “” with specified policies [always]: invalid reference format (manager.go:203:0s)

I have tried to use the dependency proxy URL manually in the runner EC2 instance, which results in the same error. Removing the port number results in “no Host in request URL” error.

Can anyone advise on how to investigate/fix this issue please?