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 Job ID Name Coverage
  Build Docker
passed #1082249
build_docker_from_cache

00:02:40

 
  Test
passed #1082251
test_mpi

00:04:36

passed #1082250
test_serial

00:05:30

83.0%
 
  Demo Runs
passed #1082256
run_bernoulli

00:00:28

passed #1082257
run_curve_fitting

00:00:26

passed #1082253
run_getting_started_1

00:00:33

passed #1082254
run_getting_started_2

00:00:56

passed #1082255
run_getting_started_3

00:01:03

passed #1082252
run_ipynb

00:01:24