docker: add commentary to debian-bootstrap.docker

This is just a note that later versions of debootstrap don't
technically need this hack.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/dockerfiles/debian-bootstrap.docker

@@ -9,6 +9,7 @@ FROM scratch
 ADD . /
 
 # Patch all mounts as docker already has stuff set up
+# (this is not needed for later debootstraps but is harmless atm)
 RUN sed -i 's/in_target mount/echo not for docker in_target mount/g' /debootstrap/functions
 
 # Run stage 2