PR previews

[3mcd]

Issue(s) Resolved

N/A

High-level Explanation of PR

This PR adds a deploy-preview step to our on_pr GitHub actions workflow that will, when labeled with a new preview label, deploy the containers produced by the upstream build-all step to temporary AWS Lightsail instances.

The changes are essentially all additive:

• A new step was added to on_pr that uses the pullpreview action
• A new docker compose file (docker-compose.preview.yml) containing simple config changes was added. This file is not meant to be used on its own, and instead is merged into the self-host docker compose file, so we're dogfooding our self hosting setup!
• pullpreview is configured to fetch images from our private ECR repository so we don't need to pollute the public GCR repository with review images

Test Plan

Find the deployment close to the bottom of this page, open it, and confirm the app works!