Add Dockerfile

[ruffsl]

This PR adds an example Dockerfile for building this package as well as any needed underlay repos.

[ToniRV]

What is the point of having an overlay/underlay.repos ?
Also we should document this in the Readme.md as well.

[ruffsl]

Having separate overlay and underlay workspaces allows one to avoid having to rebuild unchanged upstream dependencies that changed downstream packages rely on. You can read more on this here:
https://index.ros.org/doc/ros2/Tutorials/Colcon-Tutorial/#source-an-underlay

This is used in conjunction with docker layer caching to succinctly build arbitrary package dependencies in whatever order they may require, and still cache that workspace to be independent of the source package in development.

And perhaps more than you'd ever need to know about build tools:
http://design.ros2.org/articles/build_tool.html

[ruffsl]

I'll push a commit with some updated build instructions.

[ruffsl]

@ToniRV is there anything else this PR needs for to be merged?
I'd like to reduce the number patches/forks to track for the ROS2 port.

[ToniRV]

@ruffsl I need to hold these PRs bcs we are in the middle of the Go-SEEK challenge, and this might break things...