security: sourcegraph/syntax-highliter container runs as root

Created by: keegancsmith

Similiar to https://github.com/sourcegraph/sourcegraph/issues/13237 this container is unlike most sourcegraph containers and runs as root instead of the sourcegraph user. It doesn't seem like it needs to do this and is a quick security win.

This is a non-critical security issue. Varun I assigned ya since I saw you are the last person to touch this, but likely TJ is also a good candidate from my outsider perspective :)