Run alpr with Docker

[leihuayi]

Allow users to use Docker-compose for running alpr-unconstrained. Changes :

• add Dockerfile which creates an environment with python2.7, TF, Keras, opencv installed, darnknet compiled and models downloaded by only running the command: docker-compose build.
• add docker-compose.yaml file which makes the usage of Docker easier (the options for launching the container, like GPU usage and volumes, are written inside the docker-compose file rather than writing a huge command line for launching the container)
• write indications in the Readme for how to use these tools.