[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