release: Increase NJOBS in Dockerfile

5 jobs for master in 354 minutes and 35 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed job_clang_amd64 #4675
amd64

04:19:50

passed job_gcc_amd64 #4655
amd64

01:34:45

failed job_clang_amd64 #4656
amd64

01:16:26

canceled job_clang_amd64 #4657
amd64

00:18:05

failed job_clang_amd64 #4658
amd64

04:48:55