[Container Registry] Docker registry cannot be accessed with a FQDN, hence cannot be used in Portainer

Summary

Docker registry from gitlab.com cannot be accessed through a FQDN, hence cannot be accessed using some of the container management tools, e.g. Portainer.

Steps to reproduce

1. Open a brand new project
2. Push an image to your registry (Step 1 and 2 done in https://gitlab.com/neil.cresswell/testproject)
3. Start a brand new Portainer instance, done in here http://ip172-18-0-44-bdgg7mchlgn000888m7g-9000.direct.labs.play-with-docker.com , but it expires every 4 hrs.
4. Register an Docker registry with registry.gitlab.com , and then try to pull the image, neil.cresswell/testproject:alpine , unable to pull it.
5. Try registering another registry with registry.gitlab.com/neil.cresswell , pull image testproject:alpine , which allows you to successfully pull the image. However, after you create an container with it, when you try to edit/duplicate it, Portainer complains The Docker registry for the <image-url> image is not registered inside Portainer, hence you cannot do anything again

Example Project

https://gitlab.com/neil.cresswell/testproject

What is the current bug behavior?

Unable to pull with FQDN

What is the expected correct behavior?

Possible to pull with FQDN

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

This bug happens on GitLab.com (If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Everything is tried in gitlab.com

Results of GitLab application Check

Everything is tried in gitlab.com

Possible fixes

Allow pulling registry using FQDN