--- a/src/Doc/System/Misc.thy Mon Apr 05 22:45:01 2021 +0200
+++ b/src/Doc/System/Misc.thy Mon Apr 05 22:46:41 2021 +0200
@@ -77,7 +77,7 @@
of \<^verbatim>\<open>apt-get install\<close> within the underlying Linux distribution.
Option \<^verbatim>\<open>-P\<close> refers to high-level package collections: \<^verbatim>\<open>X11\<close> or \<^verbatim>\<open>latex\<close> as
- provided by \<^verbatim>\<open>isabelle build_docker\<close> (assuming Ubuntu 18.04 LTS). This
+ provided by \<^verbatim>\<open>isabelle build_docker\<close> (assuming Ubuntu 20.04 LTS). This
imposes extra weight on the resulting Docker images. Note that \<^verbatim>\<open>X11\<close> will
only provide remote X11 support according to the modest GUI quality
standards of the late 1990-ies.