[feature-flags] Update Dockerfile
Summary:
Update the Dockerfile to match the convention used in other services' Dockerfiles.
Also replaces the commapp/services-base base image so we can get rid of it.
Test Plan:
docker compose up -d --build feature-flags-server
Reviewers: michal, kamil, varun, jon
Reviewed By: michal
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D9117