This just gets confusing especially as the helper function doesn't
even take into account any extra files (or the executable). Currently
the actual check just ignores them and also passes the result through
_dockerfile_preprocess so we fix that too.
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>