Here are the issues
* webapp was built with `NODE_ENV=production` but started with
`NODE_ENV=development`
* using the provided `docker-compose.travis.yml` the command `yarn run
start` would look in the wrong folder
* removed duplicate environment definition
.. but enable it by default for `docker-compose.override.yml`. If the developer
uses `docker-compose` we can safely assume that a local smtp server for
development is running.
After endless try/error I found the way to share volumes between
multiple docker-compose.ymls: You have to place those files in the same
folder. Also the import scripts must be adapted.
