Add dock build example without dotfiles

README.md

@@ -184,6 +184,8 @@ To build a docker image:
 
 ```bash
 docker build -t devenv:${ros_distro} ${build_options} -f devenv.Dockerfile .
+e.g.
+docker build -t devenv:humble --build-arg ROS_DISTRO=${ros_distro} --build-arg PACKAGES=dummy_humble.txt -f devenv.Dockerfile .
 or
 dockbuild humble --shell zsh --ws neurondones_humble --force
 ```