How do you roll back a bad release quickly?

With version control this is made easy. Version controlled source with a build pipeline creates a versioned artifact. With configuration as code, also in source control, this specifies the version of artifact to be deployed. If a bad release/artifact is discovered, updating the configuration to the 'last known good' configuration and therefore artifact, should restore service.

This rollback path is only fast if it has been exercised — practise deploying the 'last known good' tag as a drill, not for the first time during a live incident.