Container Support

4.0-dev branch already exists for pulp_docker: https://github.com/pulp/pulp_docker/tree/4.0-dev

This task is complete when

1. The plugin is discoverable by Pulp
2. CRUD for PlaceholderContentUnits, Importers, Publishers works (vanilla, without Docker specific fields)
3. The correct PyPi metadata and license is present

To make 4.0-dev branch ready for development, we should remove Pulp 2 code and documentation and populate this branch with code generated by "plugin_template" [0] (follow README instructions).

We should retain the LICENSE, COPYRIGHT and AUTHORS files, and repopulate the metadata in setup.py with the information present in the old setup.py. We also need to replace the README e.g. [1]

[0] https://github.com/pulp/plugin_template
[1] https://github.com/pulp/pulp_python/blob/3.0-dev/README.rst