Building Docker image fails with no acceptable C compiler found

I've tried building the Docker image, which failed with the following message. This happens during the step where the Python requirements are installed.

```
configure: error: in `/tmp/pip-build-mmeastyv/gevent/deps/libev':
configure: error: no acceptable C compiler found in $PATH
```

Attached you can find the full log output from the beginning of the step.
I guess this is because the newer Ubuntu Docker images are smaller than before. I'm sure they dropped everything from the image that is not needed.

I'm currently trying to fix this issue and prepare a pull request afterwards.

[docker_build.log](https://github.com/GIScience/openelevationservice/files/4917207/docker_build.log)