Create root .dockerignore file

Summary: I'm Dockerizing the Node.js keyserver, which requires including files from various Yarn Workspaces. As a result I'm creating a .dockerignore in the root folder, and I figured it would make sure to merge all the .dockerignores into one. (Open to other approaches if people disagree.)

Test Plan: Make sure I can build the Node.js keyserver from Docker. Also CI

Reviewers: palys-swm, jimpo, varun, geekbrother

Reviewed By: varun

Subscribers: Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3710