add pip retries. should help with github's occasional flaky network during build/test (#2506)

backend/Dockerfile.model_server

@@ -15,7 +15,10 @@ ENV DANSWER_VERSION=${DANSWER_VERSION} \
 RUN echo "DANSWER_VERSION: ${DANSWER_VERSION}"
 
 COPY ./requirements/model_server.txt /tmp/requirements.txt
-RUN pip install --no-cache-dir --upgrade -r /tmp/requirements.txt
+RUN pip install --no-cache-dir --upgrade \
+        --retries 5 \
+        --timeout 30 \
+        -r /tmp/requirements.txt
 
 RUN apt-get remove -y --allow-remove-essential perl-base && \
     apt-get autoremove -y