[help.gnome.org] Install docbook-dtds, else docbook files will block on the network

[Shaun McCance <shaunm src gnome org>]

commit 0e640299e71be430ffae16b9a80fb1ffffd68da8
Author: Shaun McCance <shaunm redhat com>
Date:   Sat Nov 21 10:21:34 2020 -0500

    Install docbook-dtds, else docbook files will block on the network

 Dockerfile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/Dockerfile b/Dockerfile
index 040db2a..28cb207 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,7 +1,7 @@
 FROM centos:8
 
 RUN dnf install -y git python3-lxml itstool python3 which automake autoconf \
-        diffutils gcc gettext gettext-devel
+        diffutils gcc gettext gettext-devel docbook-dtds
 
 RUN mkdir -p /usr/local/lib/python3.6/site-packages/