[keyserver] Add facts/secrets folder to .dockerignore

Summary: Following D4102 and D4103, these values get passed into the Docker container via environmental variables. We should avoid any confusion here, and avoid accidentally leaking any secrets.

Test Plan:
Create a keyserver/.env file, eg.:

COMM_MYSQL_DATABASE=commdev
COMM_MYSQL_USER=commdev
COMM_MYSQL_PASSWORD=pass

And then make sure the Docker image still works: docker-compose down -v && docker-compose up --build

Reviewers: palys-swm, atul

Reviewed By: palys-swm, atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4150