Summary:
This is an effort to clean up the root of the repository. Everything related to Docker is being moved to the `.circleci` directory, as these images are used as part of the Circle CI 2.0 tests on Android.

[GENERAL] [Changed] - Moved Dockerfiles into .circleci directory
Pull Request resolved: https://github.com/facebook/react-native/pull/23677

Differential Revision: D14253735

Pulled By: cpojer

fbshipit-source-id: 4d482d531a71b8259b09c001b310d82ce5349b87