Pin docker version to 19.03.8

Our CI machine runs a Docker version which is too old otherwise. Hopefully to be
fixed in the near future
9 jobs for docker_fix_backport in 9 minutes and 34 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build Docker
passed build_docker_from_cache #1082249

00:02:40

 
  Test
passed test_mpi #1082251

00:04:36

passed test_serial #1082250

00:05:30

83.0%
 
  Demo Runs
passed run_bernoulli #1082256

00:00:28

passed run_curve_fitting #1082257

00:00:26

passed run_getting_started_1 #1082253

00:00:33

passed run_getting_started_2 #1082254

00:00:56

passed run_getting_started_3 #1082255

00:01:03

passed run_ipynb #1082252

00:01:24