Set Gerrit to 2.15.21

Change-Id: Iedca9fc2cbdcaca8e929cf56d02a04d5aa292a6a
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index 7696981..76a4a95 100644
--- a/centos/7/Dockerfile
+++ b/centos/7/Dockerfile
@@ -9,7 +9,7 @@
 # Install OpenJDK and Gerrit in two subsequent transactions
 # (pre-trans Gerrit script needs to have access to the Java command)
 RUN yum -y install java-1.8.0-openjdk
-RUN yum -y install gerrit-2.15.20-1  && rm -f /var/gerrit/logs/*
+RUN yum -y install gerrit-2.15.21-1  && rm -f /var/gerrit/logs/*
 
 USER gerrit
 RUN java -jar /var/gerrit/bin/gerrit.war init --batch --install-all-plugins -d /var/gerrit
diff --git a/ubuntu/16/Dockerfile b/ubuntu/16/Dockerfile
index 0246fb9..b1747f6 100644
--- a/ubuntu/16/Dockerfile
+++ b/ubuntu/16/Dockerfile
@@ -12,7 +12,7 @@
 # Install OpenJDK and Gerrit in two subsequent transactions
 # (pre-trans Gerrit script needs to have access to the Java command)
 RUN apt-get -y install openjdk-8-jdk
-RUN apt-get -y install gerrit=2.15.20-1 && rm -f /var/gerrit/logs/*
+RUN apt-get -y install gerrit=2.15.21-1 && rm -f /var/gerrit/logs/*
 
 USER gerrit
 RUN java -jar /var/gerrit/bin/gerrit.war init --batch --install-all-plugins -d /var/gerrit