Align Docker image release workflow with Postgres

For our semi-managed services, we want images to be generated automatically from Github tags. Postgres is [already setup for this](https://github.com/fly-apps/postgres-standalone/blob/master/.github/workflows/ci.yaml). We should setup this repo the same way.

This helps people track Redis versions with `fly image`. `fly image` is backed by our API which will automatically track new versions from specific repositories. The tracking is already setup for this repository in the Fly backend.