Do not update all packages on CentOS

Upgrading all packages on CentOS would have the effect
of upgrading Gerrit also, which invalidates the purpose
of using the version specific to the branch.

Change-Id: Ib4bc56664b48bebe03f649d1b4782bbab1ec9fc8
diff --git a/gerrit/Dockerfile b/gerrit/Dockerfile
index 93051f6..3512857 100644
--- a/gerrit/Dockerfile
+++ b/gerrit/Dockerfile
@@ -3,7 +3,6 @@
 USER root
 
 RUN yum install -y https://centos7.iuscommunity.org/ius-release.rpm \
-    && yum update -y \
     && yum install -y python36u python36u-libs python36u-devel python36u-pip
 
 COPY --chown=gerrit:gerrit ssh-config /var/gerrit/.ssh/config