Portable Wordcount on Flink runner finishes but does not produce output files in DOCKER execution mode

Repro steps:

```

./gradlew :runners:flink:1.5:job-server:runShadow   
./gradlew :sdks:python:container:py3:docker
rm
-rf /tmp/py-wordcount-direct*
./gradlew :sdks:python:test-suites:portable:py35:portableWordCountBatch
 -PjobEndpoint=localhost:8099 -PenvironmentType=DOCKER
cat /tmp/py-wordcount-direct* # to verify results.

#
Output: No such file or directory

```

Switching to -PenvironmentType=LOOPBACK produces output.
The issue is reproducible both on Py2 and Py3 on master.

cc: [~altay] [~angoenka] [~ibzib] [~robertwb]

Imported from Jira [BEAM-7857](https://issues.apache.org/jira/browse/BEAM-7857). Original Jira may contain additional context.
Reported by: tvalentyn.

Start with the runners/flink/1.5/job-server:runShadow entry point and the sdks/python/test-suites/portable/py35/portableWordCountBatch task. Reproduce in DOCKER mode and compare it with LOOPBACK, checking why the expected /tmp/py-wordcount-direct* output is absent. Done means the portable Wordcount job produces its output files in DOCKER execution for both Py2 and Py3.

Written by the indexing model from the issue text.