Compatibility issue with docker-compose 1.17

The traefik labels are:

      - 'traefik.backend=knimed8_mailhog_1'
      - 'traefik.port=8025'
      - 'traefik.frontend.rule=Host:mailhog.knimed8.docker.localhost'

The should be changed to:

      traefik.backend: 'knimed8_mailhog_1'
      traefik.port: '8025'
      traefik.frontend.rule: 'Host:mailhog.knimed8.docker.localhost'