move to git 2.12 in .devcontainer for vscode

In the Docker image used by vscode (.devcontainer/Dockerfile) we setup git 2.5 which is very old (2015?). This is quite annoying if you are using extensions as gitlens, who require newer version. Here I am moving to git 2.12, which is, as a far as I know the latest available on SCLo (not sure why we are using that, @akraszna ?). Take into account that lsetup git now setup 2.39

This is targeting master, an identical MR (!62254 (merged)) is targeting 23.0