Releases: add missing artifacts

[allentiak]

Currently, the only artifact for releases is the source code.
Whereas this is certainly good for reproducibility purposes, it has no "every day" use case.

Ideally, it would be nice to add:

• A link to the corresponding Docker image from https://hub.docker.com/r/samply/blaze
• The Uberjar itself.

[alexanderkiel]

We can add a link to the Docker images, but not the Uberjar, because I don't linke people to run the Uberjar standalone.