Introducing Development Dockerfile (#3263)

* Moved Dockerfile, COPY at the end

This change should prevent re-installation of the dependencies upon
every change of the repository's contents. Typically if Docker detects
that something changed in a layer, all downstream layers are invalidated
and rebuilt.

* Moved Dockerfile back to main directory

Main dockerfile in a separate directory can cause issues with the
current CI/CD setup. This can be a good change for later.

* Introduced Dockerfile.dev, updated CONTRIBUTING

Dockerfile.dev can be used as a separate development environment for
anyone that does not wish to install the dependencies locally.