Underscore in repo names

Hi!

I faced a very weird issue. We created a project with underscore in it's path: https://gitlab.com/nobox_/serverless Then - when I tried to build a docker image with name registry.gitlab.com/nobox_/serverless/python:latest I got an error:

invalid argument "registry.gitlab.com/nobox_/serverless/python:latest" for "-t, --tag" flag: invalid reference format
See 'docker build --help'.

In fact, I can't control the first part of docker image name, because it is offered me by GitLab itself. If I want to have another one, I need to create a new repo (let's say https://gitlab.com/nobox/serverless) and put all code there. But it is difficult and sometimes inappropriate. I have also a lot of different repos with different sets of symbols as a name, but please notice that in this case one element of path is ended with underscore symbol. I think it may be some issue with docker client itself and it has a flaw in tag validation regex, but in fact it is also issue with GitLab, because it may disallow me from creating the repos with weird names.

I checked and any other name (without underscore) is working well.