Fineract Dockerfile multi stage build

Once https://github.com/apache/fineract/pull/593 for FINERACT-760 is in, and perhaps after we have a first iteration of INFRA-18673 done, I'll try to make time to have a go at reworking the first Dockerfile originally contributed by conradsp to be "multi-stage", to separate the build from the runtime more. I've done this before in other projects, from which I can draw inspiration. This may help unblock a similar effort in INFRA-18516 by serving as an example to them.