diff -r 9efdebe24c65 -r 0ffcad1f6130 src/Pure/Tools/build_docker.scala
--- a/src/Pure/Tools/build_docker.scala	Mon Mar 01 20:12:09 2021 +0100
+++ b/src/Pure/Tools/build_docker.scala	Mon Mar 01 22:22:12 2021 +0100
@@ -31,7 +31,7 @@
     output: Option[Path] = None,
     more_packages: List[String] = Nil,
     tag: String = "",
-    verbose: Boolean = false)
+    verbose: Boolean = false): Unit =
   {
     val isabelle_name =
       app_archive match {