tc: update pulled docker image verification logic

If a host has two docker images of same name with different SHAs then
run_docker.sh, invoked via setup.sh, fails while verifying the pulled
docker image. This is becuase two docker images of same name but
different SHAs breaks the comparison if condition as right side
of the conditon has both the SHAs in it. This fix is now extracting
the SHA information associated with docker image name as well as
its unique TAG value.

Also, updated ubuntu version tag to fix errors while building
docker image locally.

Signed-off-by: Rupinderjit Singh <rupinderjit.singh@arm.com>