util/docker: Update makefile with command to kill docker images

Kill running docker containers before trying to remove images or
containers.

Change-Id: Id2de90edbe5d0dc6ecb906be7101ad9744dbd11e
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/17999
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
1 file changed