Improve configuration of dev container .gitconfig

Thank you for responding @chrmarti, I appreciate you discussing this even if you didn't necessarily agree with it.

I really appreciate the tip regarding devcontainer.json. That is non-obvious but it works great!

I still question why the local configuration is imported by default, but with a working shippable setting to disable that behaviour, I'm good.

I'm now running the following shell script as my postCreateCommand:

git config --global core.editor "code --wait"
git config --global diff.tool vscode
git config --global difftool.vscode.cmd "code --wait --diff \$LOCAL \$REMOTE"
git config --global merge.tool vscode
git config --global mergetool.vscode.cmd "code --wait \$MERGED"
yarn install

Maybe in the future this extension would consider setting more than just credential.helper by default; "text editor sets itself as text editor" seems like fair game in terms of behaviour, and would probably improve ease of use for most folks. It would also overwrite any incompatible imported settings.

Originally posted by @aaronadamsCA in https://github.com/microsoft/vscode-remote-release/issues/4603#issuecomment-792959197