CROSS_REMOTE incompatible with CROSS_CONTAINER_IN_CONTAINER?

[tyilo]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

I just spend a lot of time getting cross to work in my GitLab instance.

The GitLab runner has /var/run/docker.sock mounted, meaning that I would expect setting CROSS_CONTAINER_IN_CONTAINER=1 to make it work.
After this failed I tried setting CROSS_REMOTE=1 but that didn't change the behavior at all.

It was only after unsetting CROSS_CONTAINER_IN_CONTAINER that the CROSS_REMOTE option seemed to take effect.

• Is CROSS_REMOTE incompatible with CROSS_CONTAINER_IN_CONTAINER or am I just misunderstanding my issue?
• If so, I think we should make it an error (or show a warning) when both are set.

Describe why this would be a good inclusion for cross

This would have saved me a lot of time with trying to fix my cross build.