docker: minimise number of image layers
docker: minimise number of image layers

* Modifies the Dockerfile to minimise the number of image layers.
  (PR #4)

Co-authored-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>
Signed-off-by: Leonardo Rossi <5460ec99b71a72e7693c4088b5c2585e76e1c857@cern.ch>
Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

SamiHiltunen/invenio-software.org,SamiHiltunen/invenio-software.org,SamiHiltunen/invenio-software.org,tiborsimko/inveniosoftware.org,SamiHiltunen/invenio-software.org