gnu: services: docker: Add a debug? parameter.

* gnu/services/docker.scm (docker-configuration): Add a debug? field.
(containerd-shepherd-service): Pass the "--log-level=debug" argument when
DEBUG? is true.
(docker-shepherd-service): Pass the "--debug" and "--log-level=debug"
arguments when DEBUG? is true.
* doc/guix.texi (Miscellaneous Services): Update doc.