Dockerfile: build custom JRE using jlink

We could also try creating our own slimmed image using `jlink`. That seems to be the recommended practice these days.

_Originally posted by @Macroz in https://github.com/CSCfi/rems/pull/2913#pullrequestreview-936643839_

Start with the repository's Dockerfile and inspect how the current Java runtime image is built. Check how jlink can produce the proposed custom JRE, then verify that the resulting image builds and the application still starts. Done means a working slimmer image using the custom runtime.

Written by the indexing model from the issue text.