Simplify Dockerfiles and merge them into one Dockerfile

[okainov]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I cannot build Kiwi simply via docker build ., I don't really understand why is there buildroot image so separated from the main Dockerfile

Describe the solution you'd like

Have just one Dockerfile, potentially with multi-staged builds (... AS builder, COPY --from=builder...)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.