Expand bullet points about optimizations

Author: ruffsl

ros/content.md

@@ -133,16 +133,18 @@ CMD ["ros2", "launch", "demo_nodes_cpp", "talker_listener_launch.py"]
 
 The example above uses consists of three sequential stages. The `cacher` stage first updates the apt lists and ROS index, uses [`vcstool`](https://github.com/dirk-thomas/vcstool) to clone a demo repo into the workspace source directory, and derives build and runtime dependencies sets using [`rosdep`](https://docs.ros.org/en/rolling/Tutorials/Intermediate/Rosdep.html). The `builder` stage apt installs the derived build dependencies, sources the ROS install underlay, and compiles the source via release mode using [`colcon`](https://docs.ros.org/en/rolling/Tutorials/Beginner-Client-Libraries/Colcon-Tutorial.html). Finally, `runner` stage apt installs only runtime dependencies, copies the compiled workspace artifacts, and sets up the environment to launch the demo. Note the example consists of several subtle optimizations:
 
-- colcon 
-  - build cache if deps are unchanged
-  - build and install select packages
-- Caching apt and rosdep
-  - IO download
-  - synchronizing apt lists
-- multi-state parallelization
-- Final Image Size
-
-example uses the [`--mount`](https://docs.docker.com/engine/reference/builder/#run---mount) option to cache apt lists
+- Multi Stage Build
+  - Dependency derivation, compilation, and runtime setup are partitioned
+  - Maximums cache retention despite package source or build/runtime changes
+  - Greater concurrency, e.g: colcon build while runtime apt instals
+- Persistent Cache Propagation
+  - Use of [`--mount`](https://docs.docker.com/engine/reference/builder/#run---mount) to cache temp data without bloating layers
+  - Maintain temporally consistent apt lists between parallel stages
+  - Avoid needless network IO between stages or across docker rebuilds
+- Minimal Image Size
+  - Final stage builds from `ros-core` for smallest runtime image
+  - Builds and installs for just selected packages amongst workspace
+  - Only workspace install artifacts are copied into final layers
 
 For more advance examples such as daisy chaining multiple overlay workspaces to improve caching of docker image build layers, using tools such as ccache to accelerate compilation with colcon, or using buildkit to save build time and bandwidth even when dependencies change, the project `Dockerfile`s in the [Navigation2](https://github.com/ros-planning/navigation2) repo are excellent resources.