'make -j1 docker-test-build' uses more than one job

version: v4.1.0-rc5

Run 'make -j1 docker-test-build', wait a few, various containers get instantiated.

$ make -j1 docker-test-build 2>&1 > /dev/null

On another terminal:

$ docker ps
CONTAINER ID        IMAGE                            COMMAND                  CREATED             STATUS
62264a2d777a        qemu:debian-mips-cross           "/var/tmp/qemu/run t…"   10 minutes ago      Up 10 minutes
80807c47d0df        qemu:debian-armel-cross          "/var/tmp/qemu/run t…"   10 minutes ago      Up 10 minutes
06027b5dfd4a        qemu:debian-amd64                "/var/tmp/qemu/run t…"   10 minutes ago      Up 10 minutes