Use docker compose (v2) instead of docker-compose (v1)

Let's get with the times.

• Use "docker compose" instead of "docker-compose".

• Remove the 'version' key from exp/docker-compose*.yml files because
  newer versions of the docker compose plugin generate the following
  warning:

  WARN[0000] .../train-dev/exp/docker-compose.yml: version is obsolete
  WARN[0000] .../train-dev/exp/docker-compose-override-for-bind-mounts.yml: version is obsolete