[electron-update-server] Specify platform in docker compose

Summary: We need to specify the platform key in the docker compose file so that we build images for the correct platform for the aws ECS.

Test Plan: docker compose build, check if the image was built

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10196