Verifier docker image support for custom stubs classifier

Adds the possibility to provide custom stubs classifier. Might be useful to serve as some kind of tag for the stubs published.

docker/spring-cloud-contract-docker/project/build.gradle

@@ -60,6 +60,7 @@ test {
 contracts {
 	baseClassForTests = "contracts.RestBase"
 	testMode = "EXPLICIT"
+	stubsSuffix = getProp("PRODUCER_STUBS_CLASSIFIER") ?: "stubs"
 	if (getProp("EXTERNAL_CONTRACTS_ARTIFACT_ID")) {
 		logger.
 				lifecycle("Will use an artifact with contracts [${getProp("EXTERNAL_CONTRACTS_GROUP_ID")}:${getProp("EXTERNAL_CONTRACTS_ARTIFACT_ID")}]")