docker: image slim-down

• BETTER Slims down docker image by building on top of less bloated base image and only install what is really required. Also purges unneeded packages, flushes caches and clean temporary files. All these parts should not be in a production image and are also not required by developers. You can still install components when extending the Invenio base image.

• Removes dpkg package docs from the production image because admins should not use a slim docker image as a knowledge base.

Signed-off-by: Marco Neumann <marco@crepererum.net>