Fix and tidy the Docker files.

Created by: jamesturner246

Fixed and tidied up the old Docker files. Also updated the readme docker instructions.

Should work simply by building the image:

docker build --tag freedmr .

and starting it, supplying a local cfg file.

docker run -it -v /absolute/path/to/hblink.cfg:/opt/freedmr/hblink.cfg -p 54000:54000 freedmr