These roles now obey
`devture_systemd_docker_base_container_image_pull_method` and `devture_systemd_docker_base_container_network_creation_method`
and can work on systems which don't have the Docker SDK for Python
installed by avoiding the various Ansible Docker modules and using raw
`docker` commands for pulling images and creating networks.

With this change, all dependency roles are downloaded
using the same mechanism (git), which makes life simpler for tools like
https://gitlab.com/etke.cc/int/agru

With these changes, we:

- install https://github.com/Tecnativa/docker-socket-proxy via the
  https://github.com/devture/com.devture.ansible.role.container_socket_proxy Ansible role

- make Traefik access the Docker API via TCP by connecting to this
  socket proxy

- .. which allows us to run the Traefik container with less privileges
  (non-`root`, dropped capabilities)