Update navigation2 docker README after adding script for pinning repos (

space-ros#142).

Describe the image content, repo update procedure and how to facilitate script for custom scenarios

navigation2/README.md

@@ -1,7 +1,9 @@
 # Navigation2 Docker Image
 
 The Navigation2 Docker image uses the Space ROS docker image (*osrf/space-ros:latest*) as its base image.
-The Navigation2 Dockerfile installs all of the prerequisite system dependencies to build Navigation2
+The Navigation2 Dockerfile installs all of the prerequisite system dependencies to build Navigation2.
+All the required nav2 packages are cloned and built from sources using the `navigation2.repos` file. The `nav2_rviz_plugins` and `nav2_bringup` packages together with their dependencies (Rviz, Gazebo, etc) are excluded to reduce package dependencies. The image also contains a custom `space_nav2_bringup` package that can be used as a starting point for creating mission-specific nav2 launch files.
+
 ## Building the Navigation2 Image
 
 To build the docker image, run:
@@ -25,8 +27,7 @@ The output will look something like this:
 ```
 REPOSITORY              TAG                        IMAGE ID       CREATED        SIZE
 osrf/space_nav2         latest                     6edb2edc9643   10 hours ago   15.5GB
-openrobotics/spaceros   latest                     629b13cf7b74   12 hours ago   7.8GB
-nvidia/cudagl           11.4.1-devel-ubuntu20.04   336416dfcbba   1 week ago     5.35GB
+osrf/space-ros           latest                     629b13cf7b74   12 hours ago   7.8GB
 ```
 
 The new image is named **osrf/space_nav2:latest**.
@@ -37,13 +38,28 @@ There is a run.sh script provided for convenience that will run the spaceros ima
 ./run.sh
 ```
 
-Upon startup, the container automatically runs the entrypoint.sh script, which sources the MoveIt2 and Space ROS environment files.
+Upon startup, the container automatically runs the entrypoint.sh script, which sources the Nav2 and Space ROS environment files.
 You'll now be running inside the container and should see a prompt similar to this:
 
 ```
-root@ip-your-ip-address:/home/spaceros-user/nav2_ws#
+spaceros-user@ip-your-ip-address:/home/spaceros-user/nav2_ws#
 ```
 
 ## Running Navigation2 Demo
 
-To run the latest demo, see the README in the nav2_demos folder
+To run the latest demo, see the README in the [nav2_demo](../nav2_demo/README.md) folder.
+
+
+## Updating navigation2 packages
+The `navigation2.repos` file available in this repository provides a list of repos that are required to build the nav2 stack. ROS packages already included in the SpaceROS base image are omited from this file.
+
+To update navigation2 packages used to build the space nav2 image, run the `docker_update_nav2_repos.sh` script:
+
+```
+./docker_update_nav2_repos.sh
+```
+
+This will update the `navigation2.repos` file with the newest versions of nav2 packages. The script resolves missing packages in base spaceros image and creates a list with their latest versions. The `navigation2.repos` file is then copied during build to the docker image with space nav2, and packages specified in the file are cloned and installed.
+
+To generate a list of required packages for any other ROS workspace (e.g. your custom workspace built on top of SpaceROS that already has some nav2 dependencies installed), consider using the `update_nav2_repos.sh` as your starting point. You may also need to clone `generate-repos.sh` script from the main spaceros repository.
+