Add Docker config files to .dockerignore

Summary:
I was thinking of adding keyserver/Dockerfile and keyserver/docker-compose.yml to .dockerignore, so that changes to those files don't get detected by Docker while copying.

Then I figured maybe we want this for the whole project? I don't imagine we'll want these files copied into a Docker build, but I might be wrong.

Test Plan: CI :)

Reviewers: bartek

Reviewed By: bartek

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9588