Container image is built and executed correctly but step commands run outside container.
Jenkins 2.121.1 (running as a systemd unit on master and a remote build node through SSH Slave plugin)
Docker Pipeline plugin 1.17 (released in May 2018)
Docker CE 18.03 running on CentOS 7.5
And then the test stage doesn't run inside the Docker container that was started:
Running the Docker image that Jenkins built works just fine: