containers: support --root argument in build.sh and publish.sh for external context

Enhances both scripts to accept an optional --root argument
that overrides the default build context root.

Previously, the script assumed the root directory based on its
location in the repository. This change allows callers (such as CI
pipelines) to specify an alternate root directory explicitly,
enabling support for container builds from relocated Dockerfile
paths.

This improves flexibility for downstream or private repos that
maintain their own Dockerfiles independently of upstream Fastpath.

Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>