[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