Install dependencies for ml-agents-envs and ml-agents in Docker (#1895)

Dockerfile

 # xvfb is used to do CPU based rendering of Unity
 RUN apt-get install -y xvfb
 
+# Install ml-agents-envs package locally
+COPY ml-agents-envs /ml-agents-envs
+WORKDIR /ml-agents-envs
+RUN pip install -e .
+
+# Install ml-agents package next
-RUN pip install .
+RUN pip install -e .
 
 # port 5005 is the port used in in Editor training.
 EXPOSE 5005