Docker build with git source fails

I’m trying to do:

docker build

This fails with:

unable to prepare context: unable to find 'git': exec: "git": executable file not found in $PATH

This makes sense as git isn’t installed. Is there a way to install git into the system container or make it otherwise available to docker?

sudo ros --version
version v1.5.5 from os image rancher/os:v1.5.5