If you have a Jenkinsfile with a top-level agent { docker {}} section you and use parallel {} around nested stages, all the stages will run on the same slave, at the same time (even if the slave has only one executor).

If you add agent { docker {}} to each stage, they will run on other slaves, but docker run will only be invoked once, for the top-level and never again. Docker stop will run for all stages though.

If you remove the top-level agent { docker {}} and only leave it in stages, docker run will never be invoked.