How to source setup.bash in a ROS2 Dockerfile?

edit

It seems that sourcing setup.bash in a ros2 dockerfile is not that easy. In particular,

RUN bash -c "source /opt/ros/${ROS_DISTRO}/setup.bash \ && echo ${AMENT_PREFIX_PATH}"

does not print anything although AMENT_PREFX_PATH got a value from setup.bash. I have to use the RUN [.., ..] syntax. Is there an easier way to do so?

FROM ros:eloquent 

#this one does not work    
RUN bash -c "source /opt/ros/${ROS_DISTRO}/setup.bash \
        &&  echo ${AMENT_PREFIX_PATH}"

#this one works, printing /opt/ros/eloquent
RUN ["/bin/bash", "-c", "source /opt/ros/${ROS_DISTRO}/setup.bash &&  echo ${AMENT_PREFIX_PATH}"]