Upgrade CentOS to 7.7.1908

Change-Id: I1729a4ca307635b34ae94845c05339abfb7cc593
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index 1a464fa..0428cdf 100644
--- a/centos/7/Dockerfile
+++ b/centos/7/Dockerfile
@@ -1,4 +1,4 @@
-FROM centos:7.6.1810
+FROM centos:7.7.1908
 MAINTAINER Gerrit Code Review Community
 
 # Add Gerrit packages repository