docker: better usage of build cache
docker: better usage of build cache

* Reorders Dockerfile instructions to put "ADD code base" statement very
  late in the build process chain, so that one can profit from the build
  cache (for both OS base packages and for Python dependencies) when
  rebuilding the image after the code base changes.

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