Refactor docker images

[bvavala]

The base-dev and dev images can be refactored/simplified, possibly removing the latter.

Currently, we publish base-dev and the user then builds dev.
Their main difference is the presence of Fabric's source code and an option to add apt-get packages in the latter.
See the discussion here #612 (comment).

By moving (the check out of) Fabric's code to base-dev, the image is also able to test FPC -- reason is that FPC tests currently have dependencies on Fabric's source code.

Hence, dev would remain an image that only provides an option for adding apt-get packages, without impacting (by default) the FPC build and testing process.
Whether FPC should provide (tools to build) such an image, or leave this to the end-user/developer, is an open question.