Rework Docker-inside-Docker for JobServer container

Docker-inside-Docker causes problems with permissions. For example:

```

Caused by: java.lang.Exception: The user defined 'open()' method caused an exception: java.io.IOException:
Cannot run program "docker": error=13, Permission denied
at org.apache.flink.runtime.operators.BatchTask.run(BatchTask.java:498)
at
org.apache.flink.runtime.operators.BatchTask.invoke(BatchTask.java:368)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:712)
...
1 more
Caused by: org.apache.beam.repackaged.beam_runners_java_fn_execution.com.google.common.util.concurrent.UncheckedExecutionException:
java.io.IOException: Cannot run program "docker": error=13, Permission denied
at org.apache.beam.repackaged.beam_runners_java_fn_execution.com.google.common.cache.LocalCache$LocalLoadingCache.getUnchecked(LocalCache.java:4994)

...
7 more

```

See discussion:
https://lists.apache.org/thread.html/b2b97efaf1f4a6e283eaf92e05605f10f405e9eb9a9c41896041d0ac@%3Cdev.beam.apache.org%3E

We could get rid of Docker-inside-Docker entirely. It would be cleaner to start the SDK harness container alongside with JobServer container.

Alternatively, it should be possible to use an image with Docker pre-installed, which should fix the permissions errors which come from mounting the Docker executable of the host.

Imported from Jira [BEAM-6020](https://issues.apache.org/jira/browse/BEAM-6020). Original Jira may contain additional context.
Reported by: mxm.