[P1] Automate deployments with Github Actions

[willroberts]

Pulling this work out of #69 and #3 to reduce their scope.

We should automate the process of deploying OpenDuelyst when a new release is published on Github.

This can be done by creating an Actions workflow for the release event, and having that action publish container to ECR. If ECS is using the latest version of the container, I believe it will be able to pull it down automatically. If not, it will still be doable with a small amount of work.

[willroberts]

Deployment currently involves the following:

Git steps:

• Updating the version number in package.json and version.json
• Updating the deployed_version in terraform/staging/ecs.tf (should we use latest instead?)
• Merging a PR for the above, tagging the merge commit, and creating a new release

Steps which can be automated:

• Building new images with scripts/build_container.sh <service> <version>
• Publishing new images with scripts/publish_container.sh <service> <version> <aws ecr registry id>
• Using terraform apply to update the ECS deployed_version (and sometimes manually kicking the cluster if it doesn't have capacity to deploy automatically)
• Building the app for staging (NODE_ENV=staging, ALL_CARDS_AVAILABLE=true, FIREBASE_URL=foo, and API_URL-foo set)
• Uploading the new build to S3
• Creating a CloudFront invalidation for the new build's files, e.g. /staging/duelyst.js

[willroberts]

This is now better-documented here: https://github.com/open-duelyst/duelyst/blob/main/docs/DEPLOYING.md

This automation work can begin once we have permissions to access (read and write) Github Secrets.