Ubuntu 21.04 is broken, use docker

Author: cirosantilli

README.adoc

@@ -110,7 +110,11 @@ Related: <<soft-topics>>.
 
 ==== QEMU Buildroot setup getting started
 
-This setup has been mostly tested on Ubuntu. For other host operating systems see: xref:supported-hosts[xrefstyle=full]. For greater stability, consider using the <<release-procedure,latest release>> instead of master: https://github.com/cirosantilli/linux-kernel-module-cheat/releases
+This setup has been tested on Ubuntu 20.04.
+
+The Buildroot build is already broken on Ubuntu 21.04 onwards: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/155[], just use the <<docker>> setup in that case. We could fix it on Ubuntu 21.04, but it will break again inevitably later on.
+
+For other host operating systems see: xref:supported-hosts[xrefstyle=full].
 
 Reserve 12Gb of disk and run: