Changes to the Dockerfile to improve readability

This merge request contains changes to the Dockerfile to improve readability and follow best practices for docker. It also adds the --no-install-recommends flag for apt-get install. Other than the --no-install-recommends flag, the functionality or way the docker image builds should not have changed.

Splitting up some of the steps in the dockerfile using the WORKDIR directive, should not increase the size of the resulting docker image in my experience. It however enables docker to cache more of the in between steps, for when you work on the Dockerfile and make changes, that might break the build.