add NOCACHE arg

docker/Dockerfile

@@ -11,6 +11,9 @@ RUN apt-get install -yq --no-install-suggests --no-install-recommends \
     python-all-dev python-msgpack python-pip python-setuptools \
     git
 
+## Do not use cache when building next layers of the image.
+ARG NOCACHE=0
+
 WORKDIR /root/PyBitmessage
 RUN git clone https://github.com/Bitmessage/PyBitmessage .