ff1a955 pcr: Add preliminary docker-registry 2.8.1 package

Authored and Committed by Denis 'GNUtoo' Carikli a year ago
    pcr: Add preliminary docker-registry 2.8.1 package
    
    This package is based on the Guix docker-registry package that
    was made from scratch by me (and that is pending review).
    
    A basic configuration without authentication[1] was tested and works
    fine.
    
    [1] I took config-example.yml and removed the authentication part,
        and run it on localhost, and I managed to push a docker image
        to it.
    
    Note that at the moment it still lacks a systemd unit file and
    I also still need to investigate how to deploy it in production
    (so with authentication, a public domain, with TLS, etc) and to
    document that along the way on the Libreplanet Wiki[1].
    
    [1]https://libreplanet.org/wiki/Group:Software/research/ExternalRepositories/DockerRegistries
    
    When that is done we could use it for different use cases, for
    instance:
    - For setting up an official Parabola docker registry for hosting
      Parabola official images.
    - For hosting an FSDG compliant docker registry for multiple
      FSDG compliant distributions.
    - For hosting FSDG compliant docker registries for other uses (like
      personal uses, specific use cases, etc)
    
    At the moment it's possible to build docker images for the following
    FSDG compliant distributions[1]:
    - Guix
    - Parabola
    - PureOS
    - Trisquel (9 and 10)
    
    [1]https://libreplanet.org/wiki/Group:Software/research/DistroExecutionEnvironments
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>