Flink runner build instructions need updating

Hi,

I'm new to the project and trying to follow the instructions on [https://beam.apache.org/documentation/runners/flink/](https://beam.apache.org/documentation/runners/flink/) for the portable flink runner, but they are out of date. I'd love to see the docs updated so that users like myself can do this successfully.

The first issue is that all of the source releases that I downloaded, from both github and the downloads page, had the gradlew file removed.  I had to clone the repo to solve this problem.

The next problem that I encountered was this:
```

chad$ ./gradlew :sdks:python:container:docker
Configuration on demand is an incubating feature.
FAILURE:
Build failed with an exception.
* What went wrong:
Project 'sdks' not found in root project 'beam'.

```

  

The proper command is: 
```

./gradlew :beam-sdks-python-container:docker
```

 

Likewise, the next command should be:
```

./gradlew :beam-runners-flink-1.7-job-server:runShadow -PflinkMasterUrl=localhost:8081
```

  

After that I was able to get the flink beam example working.  I recommend adding this to the docs:
```

(.venv) chad$ python -m apache_beam.examples.flink.flink_streaming_impulse
```

 

also, since the docs specify only flink 1.7 is compatible with beam 2.12 (the latest as of this writing), I had to do a little extra work to get flink 1.7 built instead of 1.8, but this is probably outside the scope of the beam docs:
```

brew install https://raw.github.com/Homebrew/homebrew-core/97b7a467beb81412bef2fbc497671c8ad3d407e5/Formula/apache-flink.rb
/usr/local/Cellar/apache-flink/1.7.2/libexec/bin/start-cluster.sh
```

thanks!

Imported from Jira [BEAM-7379](https://issues.apache.org/jira/browse/BEAM-7379). Original Jira may contain additional context.
Reported by: chadrik.