[keyserver] Run Docker as non-root user

Summary:
See Linear task

Depends on D4177

Test Plan: Make sure docker-compose down -v && docker-compose up --build still works

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: varun, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4178