Add cython to Dockerfile such that pyfftw can be installed

3 jobs for nifty4_add_cython in 11 minutes and 3 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build Docker
passed build_docker_from_cache #695260

00:09:48

 
  Test
passed test_python2_with_coverage #695261

00:01:14

80.0%
passed test_python3 #695262

00:01:13