[services][identity] Add .dockerignore file for Identity Service

Summary:
We don't need to copy some of the files from the identity directory

Depends on D3593

Test Plan: built, ran docker container

Reviewers: jimpo, karol-bisztyga, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3661