docker-compose file v2 (docker-compose v1.6) compatibility

[jeroenj]

Not an urgent one for me, but just adding it as it probably will be needed at some point. Docker-compose 1.6 still supports the old format of the docker-compose.yml format.

References:

• https://blog.docker.com/2016/02/compose-1-6/
• https://docs.docker.com/compose/compose-file/#upgrading

[apeschel]

:+1:

[mauricioklein]

Improvement provided by PR#26.

Thanks to @apeschel for the contribution :+1:

[jeroenj]

Cool. :+1: