docker: Dockerfile optimizations

• BETTER Reduces number of layers required to build the Invenio Docker image.

• Reorders some instructions in the Dockerfile to optimize caching and to reduce build times.

• BETTER Add port exposure to Docker images, so they can linked together with other components like webservers, proxies, content caches etc.

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