[Identity] Avoid secret generation and slim down size of identity server docker image

Summary:
Secrets should not be generated as part of the docker image build. Instead, they need to be mounted in to the container.

Also reduces the image size from 1.81GB to 108MB.

https://linear.app/comm/issue/ENG-4419

Test Plan:

docker build -f services/identity/Dockerfile .
# or
cd services
docker compose build identity

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8580