Re-organize Dockerfile layers, refs #13441
Re-organize Dockerfile layers, refs #13441

- Install OS and composer dependencies before copying the source code to
  avoid the invalidation of the those cached layers when the container
  is built after any code change.
- Add fcgi package to allow a Docker healthcheck to be defined.
- Fix indentation.

artefactual/atom,artefactual/atom,artefactual/atom