Add an ability to specify registry when using docker images

[mathbunnyru]

Describe your changes

Work on: jupyterhub/team-compass#688

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

This doesn't change the registry to quay yet, but rather gives an ability to specify the registry.
I would like to merge this change separately - if we ever decide to change registry again, it will be simple.
After merging this, changing registry should only require only a few lines of code change.

[minrk]

Amazing, great work!