fix for failed fetch of one of the tools on this line

Author: AlexTelon

Diff for: gerrit/Dockerfile

@@ -2,7 +2,7 @@ FROM gerritforge/gerrit-ubuntu15.04:2.11.3
 MAINTAINER svanoort
 
 USER root
-RUN apt-get install -y curl python net-tools && \
+RUN apt-get update && apt-get install -y curl python net-tools && \
     rm -rf /var/lib/apt/lists/*
 RUN curl https://storage.googleapis.com/git-repo-downloads/repo > /bin/repo \
     && chmod a+x /bin/repo