Change documentation to use environment variable for server_setup
secrets instead of a docker volume introduced in https://phab.comm.dev/D8641.
Also ensure that secrets directory is created before generating secrets to
avoid docker creating it as sudo user.
Addresses https://linear.app/comm/issue/ENG-4531
I actually think this is a bit too small. But that's what's in the design.
https://www.figma.com/file/L675ETKDnGaSwlpZAw4MIC/Mobile-App?node-id=351%3A455
@ashoat - I think 16px looks better.