docker: more complete overlay documentation
docker: more complete overlay documentation

* BETTER Improves Docker documentation notably related to how to work
  with Invenio site overlays.

Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>
Reviewed-by: Marco Neumann <3829486b93ec44395f0b980424bae9b6fb07b7bc@crepererum.net>

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio