Challenge Submission Private Docker Check

[goldbattle]

I had my repositories on docker hub default to "Private" which prevents the challenge server from pulling.
I had to change this to default to "Public" and then make the submitted repo public manually.

It would be nice if this was checked when submitting to ensure that the user isn't submitting a private repo.
Or to only upload public repos if that is an option through their API.
https://challenges.duckietown.org/v4/humans/submissions/13925

[AndreaCensi]

Good idea, we should implement the check.

[AndreaCensi]

Tracked here https://ethidsc.atlassian.net/browse/DTSW-799 (private link)