validate: private gitlab environment images

[mvidalgarcia]

$ reana-client validate --environments

• Image existence check of public GitLab CERN images is addressed in validate: make image discovery more flexible #476.

• Private images check is still missing. One needs to create a Personal Access token in the GitLab UI to pass it as a PRIVATE-TOKEN Header to be able to access this information. This token is rather sensible information that can't be easily plugged into the reana-client so we must think of a smart solution. Possible solutions:

  • A) Add --gitlab-private flag or similar where the user can pass their private token to perform the checks.
  • B) Skip private image validation on reana-client altogether and face this on the cluster-side where we can save the REANA GitLab token as a secret.
  • C) Other ideas?

Originally posted by @mvidalgarcia in #463 (comment)

[mvidalgarcia]

As we're moving towards having a thinner client and moving all the logic to the server-side, option B) gains more relevance.