Dockerfile: remove useless comment

There is a dedicated page for Docker, we dont need to supply our own
documentation

Dockerfile

-# This is Docker file for container used on Gitlab to test
-# if all patches were applied correctly.
-# To build it you have to run in repository root:
-#   docker build -t registry.nic.cz/turris/turris-build -f .Dockerfile .
-# With built container you can push it to Gitlab with:
-#   docker push registry.nic.cz/turris/turris-build
-
 FROM debian:stable
 
 RUN \