Run login app with Gunicorn

The following discussion from !57 (merged) should be addressed:

• @maarten started a discussion: (+2 comments)

  Is this a CI-only Dockerfile, or is it intended to use in production in the future as well? Because then we need a better way to override these variables. We don't want to build a new container for each Stackspin instance (you can only set these ENV variables on build time).

  We probably also want to use a non-dev server like Gunicorn. Take a look at https://open.greenhost.net/stackspin/dashboard-backend/-/blob/main/Dockerfile for inspiration