Secured and customizable PLPython and PLR on Postgresql

As you know PLPython and PLR are untrusted language, which means only DBA could create these UDFs(very inconvenient). Moreover it's also hard to supply user specific Python/R env for different data scientists.

We are working on an open source  project to make PLPython and PLR trusted and customizable. The project is called PLContainer, which use Docker Container as the sandbox to avoid malicious user breaking Postgresql database or even the whole host machine.

Now there is a basic version which could support PLContainer on PG9.6 (Thanks krait007 and markwort for the contribution). We still have a lot of issues to make it production ready and share with more peoples. [Github umbrella project](https://github.com/greenplum-db/plcontainer/projects/1)

If you are interested in it, feel free to try it. Your suggestion and contribution will be appreciated.
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