Add capability to build with configuration file

[arknable]

Based on PR discussion here: #13 (comment)

[arknable]

@fabioDMFerreira @eyberg should we add this ?

[fabioDMFerreira]

Yes, I'm in favour of this. It's important that the user can build the image with a configuration file, so he can launch an instance from this image.

We can achieve the same result with Ops: Run, but some users may want to use the 2 steps (build + launch instance from image).