Rework: Docker images according to best practice

[Maleware]

1. Split generic COPY stackable/ into subdirectories and copy those only in layers necessary
   Currently we do things like that:

   COPY --chown=stackable:stackable hive/stackable /stackable

   We want to be a bit more precise with that.

2. We need to take care about when to create which layer in terms of what is changing often in the end, less changing in the Beginning
   We should reduce build times and avoid changing layers to often.

This will have the advantage of reducing buildtimes and only change layers when the change is really in there.

[fhennig]

that sounds like a good idea!

[lfrancke]

I think we mostly already do this.
I'm not sure how actionable this ticket really is.