[gimp] build: mention incomplete and outdated state of build/gimpbuilder-base/Dockerfile

[Michael Schumacher <schumaml src gnome org>]

commit 37d76c88762be00b90e16f3721db9159e682822c
Author: Michael Schumacher <schumaml gmx de>
Date:   Sun Sep 12 00:57:04 2021 +0200

    build: mention incomplete and outdated state of build/gimpbuilder-base/Dockerfile

 build/docker/gimpbuilder-base/Dockerfile | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/build/docker/gimpbuilder-base/Dockerfile b/build/docker/gimpbuilder-base/Dockerfile
index 867a28386c..7452dcfeb4 100644
--- a/build/docker/gimpbuilder-base/Dockerfile
+++ b/build/docker/gimpbuilder-base/Dockerfile
@@ -1,3 +1,5 @@
+# This file is an outdated incomplete experiment for a Docker-based build environment and won't get you a 
working build environment. It was created just before our switch to GitLab CI and became mostly obsolete due 
to this.
+
 # gimpbuilder-base
 
 FROM debian:testing