Try to create more slim Docker layers

5 jobs for slim_docker in 18 minutes and 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build Docker
passed #547098
build_docker_from_cache

00:06:31

 
  Test
passed #547101
test_python2_mpi

00:11:31

passed #547099
test_python2_scalar

00:02:29

65.0%
passed #547102
test_python3_mpi

00:11:07

passed #547100
test_python3_scalar

00:01:40