[Services] Make build_service.sh PWD independent

Summary:
This allows for the script to be invoked from another directory
Nice quality of docker scripts to avoid having to issue another WORKDIR.

Also should allow for dev workflow to call script from anywhere (including root)

Depends on D4684

Test Plan: Follow test plan from blob or backup diff in this stack

Reviewers: atul, max, karol, abosh

Reviewed By: max, karol, abosh

Subscribers: ashoat, tomek, adrian

Differential Revision: https://phab.comm.dev/D4685