[Docker] Update docker build

The current docker build still depends on `activator` builds that have been retired by the play framework in [2017](https://www.lightbend.com/blog/introducing-a-new-way-to-get-started-with-lightbend-technologies-and-saying-goodbye-to-activator).

Its probably a good idea to switch to pure `sbt` builds in the Docker build and upgrade the quite outdated Java Docker container version at the same time.

Start by locating the Docker build definition and any references to activator or sbt. Check the current Java container version and the existing build flow. Done means the Docker build no longer depends on activator, uses sbt, and runs with an updated Java container.

Written by the indexing model from the issue text.