Merge branch 'docker_speedup' into 'master'

Dockerfile: optimize dnspython using Cython

See merge request !10
2 jobs from master in 7 minutes and 39 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #182660
dind
update_ci_image

00:02:51

 
  Deploy
passed #182661
dind
update_prod_image

00:04:48