containers: Introduce rootless docker run feature

To prevent root privilege escalation from within a container, rootless
docker are engouraged.

This patch include script modifications to  build and launch a
rootless container along with some generic improvements

Removed hardcoded user information from build step and moved it to
be picked up at runtime when container is launched. Added entrypoint
script to have better user experience when running containers.
Improved container.sh to support rootless feature.

Signed-off-by: Ibrahim Uslan <ibrahim.uslan@arm.com>
Change-Id: I38831c84edee17f6a9bb9beebd3bc1740a97950f