Add to README instructions to avoid the lengthy building process

[awesomebytes]

To be more friendly with your users you could be more verbose with how to use your pre-built Docker images. e.g.:

# Pull your image with the corresponding DOCKER_TAG (kinetic, melodic, kinetic_32b or melodic_32b)
docker pull softbankroboticsresearch/sbre_robot_ros_gentoo_prefix:DOCKER_TAG
# Rest of your instructions to copy the system to the robot

You can also add somewhere that they can use your images as cache if they want to build extra stuff starting from your Dockerfile avoiding the lengthy process too.

# Add your changes to the Dockerfile (preferably after the end to be able to re-use the cache)

# Pull your image with the corresponding DOCKER_TAG (kinetic, melodic, kinetic_32b or melodic_32b)
docker pull softbankroboticsresearch/sbre_robot_ros_gentoo_prefix:DOCKER_TAG
# Build using the cache from that image
docker build --network host -f Dockerfile --cache-from softbankroboticsresearch/sbre_robot_ros_gentoo_prefix:DOCKER_TAG -t NAME_OF_CUSTOM_IMAGE .

Good job though! Advertise this project more :)