homogenize rocker/docker instructions

Signed-off-by: Tully Foote <[email protected]>

README.md

@@ -24,7 +24,9 @@ Optionally, you may choose to build this repository using Docker, for
 convenience.
 Make sure you have a recent version of [Docker](https://docs.docker.com/) and
 [nvidia-docker](https://docs.nvidia.com/datacenter/cloud-native/container-toolkit/install-guide.html#docker)
-installed. Next to get started simply run the following command.
+installed.
+And [rocker](https://github.com/osrf/rocker) available as debian package `python3-rocker` with the ROS packages.
+ Next to get started simply run the following command.
 ```
 docker/build_and_run_docker.sh
 ```
@@ -36,8 +38,6 @@ docker/join.sh mbari_lrauv
 . /home/colcon_ws/install/setup.bash
 ```
 
-## Rocker Alternative
-
 
 ### Empty World from Source