Updating Dockerfile to work with python directory.

Dockerfile

 RUN apt-get install -y xvfb
 
 
-COPY requirements.txt .
+COPY python/mlagents/requirements.txt .
-WORKDIR /execute
-COPY mlagents /execute/mlagents
-COPY setup.py .
+COPY python/mlagents /mlagents
+WORKDIR /mlagents
 RUN pip install .
 
 # port 5005 is the port used in in Editor training.

docs/Using-Docker.md

            -p 5005:5005 \
            balance.ball.v0.1:latest 3DBall \
            --docker-target-name=unity-volume \
+           <trainer-config-path> \
            --train \
            --run-id=3dball_first_trial
 ```