Docs should explain if the order of features matter / how dependencies between features are modelled

[fwilhe]

Reading this section, I'm wondering a few things:

• Does the order of features matter?
• In which order are they executed, left to right or right to left?
• If nginx declared base as a dependency, how would this influence the commandline?
• Assuming we add another feature postgresql which also depends on base, and I want to build an image with both nginx and postgresql included, how would I do that?

Maybe not all of those questions need to be answered in this section, but I'd appreciate if the doc would be a bit more explicit here :)