You run a git server on a public facing machine:

https://git-scm.com/book/en/v2/Git-on-the-Server-The-Protoco...

I run mine on the linode hosting my blog. The web interface is generated by stagit.

https://jeskin.net/blog/self-hosting-git-with-stagit/

Another perhaps lesser known way is to "git add origin /path/to/repo".

For example works great on networked file share with no proper git server required.

you got some good answers, just adding this as a you-should-know.

you can simply self-host git in any ssh server as long as you don't need to share access.

in the remote server just "git --bare init" a directory, in your local computer "git remote add myorigin ssh://server/path/to/dir" and start pushing.

you could also do the same thing for a dropbox directory adding a file:// remote.

git is just some files lying around, no weird binary formats or servers needed, it's brilliant

One way is to run a self hosted gitlab-ce instance in docker.