.gitignore_global doesn't work in devcontainers

`~/.gitignore_global` (defined in your host machine) doesn't work in devcontainers.

All the existing workarounds (like mounts) are terrible and they add a lot of complexity / possible bugs to something that should be straightforward and that should work out of the box for all developers in the team.

Another issue that adds more complexity is that VS Code copies the `~/.gitconfig` but it doesn't copy `~/.gitignore_global`, so all the paths defined inside `~/.gitconfig` are broken in the devcontainer.

VS Code should at least copy `~/.gitignore_global` in the dev containers as it already does for `~/.gitconfig`.