There is no way to deduce what kind of git service is being self-hosted by the url alone. Currently, it assumes GitLab self-host if it is unable to deduce what service it is. I propose adding an optional identifier to the url field when configuring custom repositories.

I have created a branch with some proposed solutions along with support for Gitea which I am using here:
https://github.com/stianKristensen/AddonManager/tree/selfhost_options

I this branch I have used the pipe key to allow for directly informing the addon manager of what service you are using.
I.e. "<repo_url>|<service_identifier>", example using gitea: "https://mydomain.com/myuser/myrepo|gitea"

When using gitea with the above changes, I have still not been able to get the icon and description to load properly when opening the addon manager.
The readme shows up fine and the description loads after installing the addon through the addon manager, the icon however still does not load.

I would like to get the icon and pre-installation description loading working (along with adding/updating related tests) before a potential pull request.