docker: add special handling for FROM:debian-%-user targets

These will have been build with debootstrap so we need to check
against the debian-bootstrap dockerfile. This does mean sticking to
debian-FOO-user as the naming conventions for boot-strapped images.
The actual cross image is built on top.

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