Adds WORKDIR command inside Dockerfiles

[pantuza]

Hi, folks.

This Pull Request reduces the number of command line arguments needed to be passed when getting started with this project/container.

It is a small contribution by adding WORKDIR inside Dockerfiles. If an user wants to make a specific directory as the work directory he can still do it by using --workdir that overrides the WORKDIR from inside the Dockerfile.

Thanks for this project!