Global gitconfig fails to copy to devcontainer with postCreateCommand

Related to this issue https://github.com/devcontainers/cli/issues/98 there still seems to be a problem for the global git config.

My local git config is copied, but my global git config is not. Settings that only existed in my global config like my name and email are not copied.

It seems that a default empty global git config is created when using `postCreateCommand`. In my case, I am running a script `"postCreateCommand": "sh ./scripts/env-setup.sh"`

Here is what my containers global git config looks like when using `postCreateCommand`:

```
# This is Git's per-user configuration file.
[user]
# Please adapt and uncomment the following lines:
# name =
# email = vscode@b97bec6ca4f1.(none)

```

If I change it to `postAttachCommand`, then an exact copy of my actual global git config file from my machine is copied correctly into the dev container.