ttcn3.sh: support IMAGE_SUFFIX=latest

Add the image suffix to the marker in run_docker() which indicates if
the testsuite docker container was already built (with all its
dependencies), so the script doesn't assume that everything is already
built if using without IMAGE_SUFFIX first and then switching to
IMAGE_SUFFIX=latest.

Add an usage example to README for IMAGE_SUFFIX=latest.

Change-Id: I867740831810324a7f5906a0548f0d04ecb9a0a1
2 files changed