yocto-build-and-test.sh: Skip Docker build on test job
yocto-build-and-test.sh: Skip Docker build on test job

The Docker images are only needed on "build only" invocation of the
script, where they will be passed to other pipeline jobs.

Signed-off-by: Lluis Campos <9244cb41c39addfc5cf236920b776ea2820513d0@northern.tech>

mendersoftware/meta-mender-qemu