Fall back to flink-conf.yaml if no JOB_MANAGER_RPC_ADDRESS is specified

Currently docker-enterypoint.sh always overrides jobmanager.rpc.address in flink-conf.yaml either with an environment variable JOB_MANAGER_RPC_ADDRESS or with a hosname : link . This causes, for instance, jobmanager address configured in FlinkContainers that are based on an existing image (contains docker-entrypoint.sh in contrast to an image that is built on the flight from flink-dist) to be overridden by the hostname. TMs then fail for connect to the JM. A workaround is to use TestContainersSettings to set JOB_MANAGER_RPC_ADDRESS, which is suboptimal from the user's perspective.

Configuration in flink-conf.yaml should instead be kept if JOB_MANAGER_RPC_ADDRESS is not passed explicitly and only overridden by the hostname if nothing was specified in the config.