Unable to pull custom tagged images from Gitlab.com Private Registry

[gualandri43]

Bug description

Portainer is unable to find or pull images from a Gitlab.com registry with a tag other than latest.

Expected behavior
Pull from Gitlab.com private registry using various tags or subdomains.

Steps to reproduce the issue:

Steps to reproduce the behavior:

1. Add Gitlab.com as registry in Portainer with auth
2. Externally push image to Gitlab.com registry with a tag other than latest (dev, newname, etc) or
   a subdomain of the main registry (registry.gitlab.com/username/projectname/subdomain)
3. Create container in Portainer using that image and registry.
4. Red error message in upper right corner: "No such image: <path/to/registry>"

Technical details:

• Portainer version: 1.20.0
• Docker version (managed by Portainer): 18.06.1-ce
• Platform (windows/linux): linux (ubuntu)
• Browser: Chrome
• Command used to start Portainer: stack compose file

services:
  portainer:
    image: portainer/portainer
    deploy:
      placement:
        constraints:
          - node.role == manager
      mode: replicated
      replicas: 1
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - portainer_data:/data
    ports:
      - 9000:9000
volumes:
  portainer_data:
    external: true

Additional context
This may be related to #2105 but is slightly different. I am able to pull images from my private Gitlab.com repository if the tag is latest and at the root of my repository url, but I am unable to pull any other sort of tag or any subdomain. I have tried multiple tag names without success.

I am able to pull and use the images from this registry using docker from the command line, so this appears to be a Portainer issue.

I originally tried to pull these images using a stack compose file, and that failed as well. When I looked into the error message, the image SHA listed did not match the SHA listed on my Gitlab registry page. It actually did not match any images on there, not just the one with the tag that I was trying to pull.

[gualandri43]

UPDATE
After further inspection, I believe the cause of my issue is not Portainer's pull, but that I ran out of space on the underlying machine hosting my Docker Engine. Therefore, new pulls weren't completing and the images being shown were of earlier/incomplete versions of the images. Trying the pull on a fresh machine worked as expected.

This cause was found by logging into the machine hosting Docker and Portainer and trying the image pull manually, receiving an out of space error message.

Portainer's error message, as stated above, indicated that the pulled image could not be found, or the wrong (old) SHA was used, which is true since the pull didn't have hard disk space to complete. However, there was no indication from Portainer of the root cause of the image pull error.

If possible, I suggest adding a more specific error notification to Portainer to help diagnose issues like this.