fvp-docker-images.yaml: introduce fvp docker images job

The new job, fvp-docker-images, is a cron-type job, but can be also
manually triggered from the  job's landing page by the TF
community.

Job calls the builders.sh script, introduced in parent
commit, which handles fvp docker images generation and image pull.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I4089f18f48ea146803dc628fac188196efe5c5cf
1 file changed