add missing lib

Author: ib-steffen

src/gc/Dockerfile

@@ -1,6 +1,6 @@
 FROM debian:9.4-slim
 
-RUN apt-get update -y && apt-get install -y python python-psycopg2 python-pip && \
+RUN apt-get update -y && apt-get install -y python python-psycopg2 python-pip python-six && \
     pip install boto3 google-cloud-storage eventlet \
     azure-mgmt-resource azure-storage keystoneauth1==3.7.0 python-swiftclient && \
     apt-get remove -y python-pip && \