Always add a user

[ffes]

This PR always adds a user git-user. The adduser command is inspired on your stale non-root branch.

My reason to always add a user, but not use, is to have the possibility to use it as a devcontainer in vscode.

I have a couple of repositories that I use to maintain my notes on various subjects. In vscode when you want to use a devcontainer as non-root, vscode requires to have that there is a user available in the container. There is no easy way to add it afterwards unless you create a new Dockerfile and use that to build your own base-devcontainer. So by always adding the user, I can use this container in vscode really easily.

I tried to pull your alpine/git:user from docker hub, but that is too old and doesn't work with vscode.

As far as I can see, adding this doesn't bother the current users who use the container as-is.