docker: Do not hardcode entry point
    
    Do not hardcode an entry point because images with systemd need
    `/usr/sbin/init` as entry point and not `/bin/sh`. Container images can
    specify the entry point in the container specification (Dockerfile). If
    it is necessary to specify it on the command line, users can use
    `TEST_DOCKER_EXTRA_ARGS` to set an entry point.