more NEWS;

--- a/NEWS	Sat Mar 21 21:12:07 2020 +0100
+++ b/NEWS	Sat Mar 21 21:41:13 2020 +0100
@@ -1496,6 +1496,9 @@
 is analogous to ISABELLE_PLATFORM, ISABELLE_PLATFORM32,
 ISABELLE_PLATFORM64.
 
+* The command-line tool "isabelle build_docker" has been slightly
+improved: it is now properly documented in the "system" manual.
+
 * Command-line tool "isabelle build_docker" builds a Docker image from
 the Isabelle application bundle for Linux. See also
 https://hub.docker.com/r/makarius/isabelle