docker: pass EXECUTABLE to build script

docker: pass EXECUTABLE to build script

To build a docker image with which needs qemu linux-user emulation we
need to pass --include-executable to the build script. Using the same
mechanism as for other container controls we enable the option is
EXECUTABLE is set on the make command line e.g:

    make docker-image-debian-bootstrap V=1 J=9 DEB_ARCH=armhf \
        DEB_TYPE=stable EXECUTABLE=./arm-linux-user/qemu-arm

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1468934445-32183-11-git-send-email-famz@redhat.com
Signed-off-by: Fam Zheng <famz@redhat.com>