feat(docker): setup github stuff to be parity with efs

[seaerchin]

NOTE: Setup changes required after this PR is merged!!!

Problem

Docker setup previously couldn't push to github due to lacking ssh creds + git config. This PR solves that issue.

Closes [insert issue #]

Solution

• copy over stuff from a local .ssh folder into docker
• add git + ssh clients
• add github to trusted hosts

Setup instructions

• This assumes that your git.config is global; if this isn't so, remedy by following the instructions here
• Copy over your ssh creds for github into a .ssh folder rooted inside our workdir (that's isomercms-backend/). Name the public key github.pub and hte private key github
• ensure that your local DB_URI is postgres://isomer:password@postgres:5432/isomercms_dev (updated alr in 1pw)