update Dockerfile

Dockerfile

 FROM pytorch/pytorch:1.1.0-cuda10.0-cudnn7.5-devel
 MAINTAINER Yaman Umuroglu <yamanu@xilinx.com>
+ARG PYTHON_VERSION=3.6
+
+WORKDIR /workdir
+RUN git clone --branch feature/onnx_exec https://github.com/Xilinx/FINN.git finn
+
+WORKDIR /workdir/finn
+RUN pip install -r requirements.txt