[identity] hard-code the platform type for the identity docker image

Summary: On M1 Macs, the default image platform is arm64. We should build x86-64 images, though, since our EC2 instances are x86-64.

Test Plan: docker-compose build identity-server builds an x86-64 image on my M1 now

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8447