[Docker] use correct brevitas branch, fix envvars

Dockerfile

@@ -4,7 +4,7 @@ ARG PYTHON_VERSION=3.6
 
 WORKDIR /workspace
 RUN git clone https://github.com/maltanar/brevitas_cnv_lfc.git
-RUN git clone https://github.com/Xilinx/brevitas
+RUN git clone --branch feature/finn_onnx_export https://github.com/Xilinx/brevitas
 RUN cd brevitas; pip install .
 
 COPY requirements.txt .
@@ -15,4 +15,4 @@ RUN rm requirements.txt
 # Note that we expect the cloned finn directory on the host to be
 # mounted on /workspace/finn -- see run-docker.sh for an example
 # of how to do this.
-ENV PYTHONPATH "${PYTHONPATH}:/workspace/finn"
+ENV PYTHONPATH "${PYTHONPATH}:/workspace/finn:/workspace/brevitas_cnv_lfc/training_scripts"