Add the commands ssh-agent and ssh-add to the base image

[deflaux]

With Terra's new support for detachable persistent disks, its now safe to checkout code from source control to our persistent disk and modify it - hurray!

For example, we might checkout https://github.com/DataBiosphere/notebooks to bring the notebooks in featured workspace Terra Notebooks Playground up to date.

Installing the keychain package provides the commands ssh-agent and ssh-add which are needed for use with GitHub ssh keys.