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
diff --git a/ttcn3/ttcn3.sh b/ttcn3/ttcn3.sh
index daa9561..b25c239 100755
--- a/ttcn3/ttcn3.sh
+++ b/ttcn3/ttcn3.sh
@@ -236,7 +236,7 @@
 	local hacks="${DIR_OSMODEV}/src/osmo-ttcn3-hacks"
 	local docker_dir="$(get_testsuite_dir_docker)"
 	local docker_name="$(basename "$docker_dir")"
-	local marker="${DIR_OSMODEV}/ttcn3/make/.docker.$docker_name"
+	local marker="${DIR_OSMODEV}/ttcn3/make/.docker.$docker_name.$IMAGE_SUFFIX"
 
 	# Skip building docker containers if this already ran
 	if [ -e "$marker" ]; then