Stable server error: docker-compose.yml is unsupported

Hm, I just merged #827 and stable seems to have failed to build...

Version in "./docker-compose.yml" is unsupported. You might be seeing this error because you're using the wrong Compose file version. Either specify a supported version (e.g "2.2" or "3.3") and place your service definitions under the `services` key, or omit the `version` key and place your service definitions at the root of the file to use version 1.
For more on the Compose file format versions, see https://docs.docker.com/compose/compose-file/
Makefile:4: recipe for target 'build' failed

Actually it seems to have failed since #803 -- @alaxalves probably something small, would you mind taking a look? Thanks a lot!

https://jenkins.laboratoriopublico.org/job/Mapknitter-Stable/200/