cmd/server: use sourcegraph/alpine base image and sourcegraph user
Created by: ggilmore
See: #2044
When migrating from the latest insiders to this version, we get the following error:
2019/02/26 19:46:34 enterprise edition
/etc/sourcegraph/ssh does not exist, so only repos that do not require SSH will be accessible.
$ mkdir -p /run/postgresql
✱ Setting up postgres failed:
$ mkdir -p /run/postgresql
mkdir: can't create directory '/run/postgresql': Permission denied
exit status 1