Add documentation for features that require internet access
We have a bunch of features that are working on on-premises or self-managed GitLab instances in the very same way they work on GitLab.com.
Anyway, some of these features require internet access to be functional. This may be because they pull docker images from Docker Hub, or they rely on external tools that are downloaded on the fly.
If they don't have proper internet access, they will not work. There are environments where no external connections are allowed (air-gapped), or simply a firewall blocks network traffic.
In order to set right expectations to users of these scenarios, and to help admins to update their environments (e.g., adding firewall rules), we should document any requirement for external resources.
List of known items that should be considered.