Have the default examples have the steps to install other DDS implementations

[christophebedard]

Or -- analogously, have the default examples have the steps

    - name: Install Tier 1 DDS
      run: sudo apt install ros-iron-rmw-cyclonedds-cpp ... -y

In them so its immediately clear. Perhaps also explicitly set the env var too to make it clear to folks the workflow to get what they want out of the box without thinking.

Originally posted by @SteveMacenski in ros-tooling/setup-ros-docker#58 (comment)