[services][identity] add identity service port to the .env file for docker-compose

Summary:
claiming 50054 for the identity service

Depends on D3578

Test Plan: this env var isn't used anywhere yet, so this is a no-op

Reviewers: jimpo, karol-bisztyga

Reviewed By: jimpo

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3590