Add README.md file for the docker directory

See issue #5873

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)

docker/README.md

@@ -0,0 +1,3 @@
+docker.go contains Docker's main function.
+
+This file provides first line CLI argument parsing and environment variable setting.