Docker upgrade

[bloodearnest]

Preparation:

• [x] communicate advance notice of window to users on #opensafely and #opensafely-users channel
• [x] set reminder for window start

When window starts:

• [x] stop job runner
• [x] gracefully kill jobs

See the preparing for reboot section of the playbook for how to do this.

When the VM is back up:

• [x] Is docker up? run docker ps on the VM
• [x] Is job-runner up? check status, or sudo systemctl status jobrunner