Dockerize python-threatexchange

[dougneal]

Summary

Reduce user friction by providing a Docker container for the CLI. This eliminates the need for the user to have to care about Python.

To make for a clean Docker image layout, the persistent state directory is now configurable. While the default remains ~/.threatexchange, it can now be overridden by setting a TX_STATEDIR environment variable.

Test Plan

$ docker build --tag threatexchange .
$ docker run threatexchange --help
usage: threatexchange [-h] [--factory-reset] [--version] [--verbose]
                      {config,fetch,match,label,dataset,hash} ...

Wrapper for the `threatexchange` library, can serve as a simple e2e solution.
...

[BarrettOlson]

Awesome! here on in another PR I think it is worth updated the README.md with details/expected usage

[dougneal]

The README changes got stuck on my machine. Fixed

[Dcallies]

I don't use a lot of docker - what does this allow that users might like?

[dougneal]

I don't use a lot of docker - what does this allow that users might like?

The value to the user is that they don't have to be Python developers to run it, or even care about their Python install at all. Just docker build followed by docker run and you have a functional CLI. The value will be even greater when you can spin up the API in the same manner.

If we were to publish the Docker image to the Docker hub or another container registry, they wouldn't even need the build step, just docker run docker.example.com/threatexchange.

[Dcallies]

So close! Just need to run black!