namely:

• use testinfra's host to run commands instead of subprocess mostly for the sake of consistency.

• use 'compose v2' command if available instead of the old 'docker-compose' tool.
  
  See https://docs.docker.com/compose/#compose-v2-and-the-new-docker-compose-command

• simplify docker_compose fixture usage a bit
  
  make it yield the docker_host on which a check_compose_output lambda has been added; this allows users of the docker_compose fixture to use it directly to run compose commands in addition to usual commands without having to explicitly use the compose_cmd fixture.