pass git credentials into container

[maxheld83]

currently, git inside the container does not know my git credentials, which can make using git inside the container hard.

Though this is a little dicey and needs to be done safely.

Also, there's the question of whether people really should be using git inside the container.

[maxheld83]

related to, but slightly different from soztag/fossos#147

[maxheld83]

these instructions for doing this on rstudio.cloud might be helpful https://maurolepore.github.io/cloudgithub/

Or maybe we just have to save this into the docker environment.
Not sure how to script this.