ci: Add a Docker image for the CI jobs

This patch introduces a Docker image (utility-image) built in the .pre
stage to be used for CI jobs.

Changelog: other
Signed-off-by: Jingzhao Ni <Jingzhao.Ni@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Filipe Rinaldi <filipe.rinaldi@arm.com>