Add docker install requirement in the docs

[paulzierep]

running planemo test/serve --biocontainers fails if docker is not installed and the user is not part of the docker group, that should be mentioned in the docs

[bgruening]

@paulzierep please try to get this documented, should be a nice first contribution to planemo.

[mvdbeek]

  --mulled_containers, --biocontainers
                                  Test tools against mulled containers (forces
                                  --docker). Disables conda resolution unless
                                  any conda option has been set explicitly.

and just above that:

  --docker / --no_docker          Run Galaxy tools in Docker if enabled.
  --docker_cmd TEXT               Command used to launch docker (defaults to
                                  docker).
  --docker_sudo / --no_docker_sudo
                                  Flag to use sudo when running docker.
  --docker_host TEXT              Docker host to target when executing docker
                                  commands (defaults to localhost).
  --docker_sudo_cmd TEXT          sudo command to use when --docker_sudo is

[paulzierep]

@mvdbeek I know that part of the docs, and probably it is clear to many people that a docker installation with user rights is required for this. But I think it is also possible that users develop tools without any knowledge of docker at all. And since planemo test --biocontainers can fail (the test is shown as failed) without giving an error message that docker is missing, it would improve UX if the info was given in the installation requirements.