[!84] Eliminate "ci" user from the built pacxx images

Merge branch 'no-ci-user-in-resulting-images' into 'master'

ref:joe/pacxx-docker It is customary for images to default to the root user

Closes: [#38]

See merge request [joe/pacxx-docker!84]

  [#38]: gitlab.dune-project.org/NoneNone/issues/38
  [joe/pacxx-docker!84]: gitlab.dune-project.org/joe/pacxx-docker/merge_requests/84


Closes #38