draft: fix: migrate away from deprecated bitnami image

[Morl99]

BREAKING CHANGE: dropping support for postgres 13
BREAKING CHANGE: switching to official postgres image with alpine base

relates to #31

[Morl99]

Since this is a breaking change, I wonder on how and when we should merge this and how we communicate this to reduce suprises by consumers.

[jschirrmacher]

At least, we should add a note in README.

What could happen in the worst case? If someone still uses version 13, they would get the old image, I suppose? How could someone rely on the bitnami base (e.g. extending our image with more packages), they would get errors and hopefully would read the README file at first.

Nice idea to use official PostgreSQL images as the base. Why did we use bitnami in the first place?

[Morl99]

Yeah, worst case it breaks Dockerfiles of people that use our image at the base or breaks deployments where people rely on Env Variables of the bitnami image.

Good question, because we used bitnami in our project and it seemed like a good choice, I guess.