Reverted Dockerfile cython to 0.28.6

Checking when this was fixed in cython, it looks like 0.28.6 should be fine

Author: inclement

Dockerfile

@@ -93,7 +93,7 @@ RUN useradd --create-home --shell /bin/bash ${USER}
 # with sudo access and no password
 RUN usermod -append --groups sudo ${USER}
 RUN echo "%sudo ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
-RUN pip install --quiet --upgrade cython==0.29
+RUN pip install --quiet --upgrade cython==0.28.6
 WORKDIR ${WORK_DIR}
 COPY . ${WORK_DIR}
 # user needs ownership/write access to these directories