Add integration tests to STAGE -> PROD deployment pipeline

[grahamalama]

We currently have integration tests set up with docker-compose that we use as a quality gate for merging PRs to main. Merges trigger an automatic deploy to the DEV environment.

After some time, we push a version tag which triggers a deployment to STAGE. We then "promote" the stage build to PROD.

Goal: Configure the STAGE -> PROD promotion pipeline to run integration tests on STAGE. Report the outcome of these integration tests to inform our decision of whether we should continue with the deployment.

For this ticket, we'll modify the Github Action workflow to use this strategy.