get rid of "WARN: InvalidDefaultArgInFrom: Default value for ARG flin…

…k:${FLINK_VERSION} results in empty or invalid base image name (line 3)"

flink/Dockerfile

@@ -1,4 +1,4 @@
-ARG FLINK_VERSION
+ARG FLINK_VERSION=undefined-flink-version
 
 FROM flink:${FLINK_VERSION}