Add docker support

[fabiomaschi]

This PR adds a docker image to make reproducibility easier regardless of the local environment setup:

• dependencies do not need to be installed in the user OS
• dependencies are clearly stated from an empty OS image
• OS, tool versions and installing scripts can be versioned
• less commands to be taken care from the user: just build and execute!

With this PR, as documented in docs/docker.md, user can simply run:

docker build -t rds:latest .
docker run -i -w /rds rds:latest ./build/baseline_crowd_rds_5 7

Tests

This PR has not been fully tested apart from the commands listed above, and no correctness check has been made with the output.

Next steps

Given the time allocated to this PR, I've just added to the docker image the main dependency (libsdl2-dev). Next steps would comprise adding ROS Kinetic and the catkin command line tools install scripts to the Dockerfile file.