The verdaccio-gitlab plugin can be used to authenticate users against gitlab.
If the only extra plugin you require is verdaccio-gitlab, you can use the image provided by the plugin project. It takes care of the whole plugin setup and is based on the official verdaccio docker image.
More information about the configuration required can be found in the verdaccio-gitlab readme.
https://hub.docker.com/r/bufferoverflow/verdaccio-gitlab/
There's a sample docker-compose file in the verdaccio-gitlab repo that shows how to build & start both gitlab and verdaccio with support for the gitlab plugin, but this is generic enough to be used with any other extra plugins.
You can also extend the official verdaccio-gitlab image Dockerfile.