[Docker] allow specifying CI branch

docker/Dockerfile.finn_ci

@@ -31,6 +31,7 @@ MAINTAINER Yaman Umuroglu <yamanu@xilinx.com>
 ARG PYTHON_VERSION=3.6
 ARG VIVADO_PATH
 ARG BUILD_PATH
+ARG FINN_CI_BRANCH
 
 WORKDIR /workspace
 
@@ -60,7 +61,7 @@ RUN git clone --branch feature/synth_rpt https://github.com/maltanar/PYNQ-HelloW
 RUN git -C /workspace/PYNQ-HelloWorld checkout db7e418767ce2a8e08fe732ddb3aa56ee79b7560
 
 # FINN
-RUN git clone --branch dev https://github.com/Xilinx/finn /workspace/finn
+RUN git clone --branch $FINN_CI_BRANCH https://github.com/Xilinx/finn /workspace/finn
 
 RUN pip install -r /workspace/finn/requirements.txt
 RUN apt update; apt install nano