security: sourcegraph/server runs as `root`

Created by: slimsag

This is a long-standing non-critical security issue, the default execution mode of sourcegraph/server is root:

$ docker run --entrypoint=sh -it sourcegraph/server:3.19.1 -c 'whoami'
root

This is different than all other Sourcegraph containers which run as a non-root sourcegraph user. Migrating all existing deployments to non-root would be complex.

In this particular case, we aim to deprecate sourcegraph/server as a deployment method in the not-too-distant future so this is merely for tracking purposes.