Make docker images thinner and use tags

[oesteban]

The current versioning of docker images is not appropriate. We should aim at having only one docker image name nipype/nipype for the production images (with all the software written in #1536) and use tags to identify the images:

nipype/nipype:py2.7
nipype/nipype:py3.4
nipype/nipype:py3.5

For tests, I would propose to have one nipype/nipype_tests. That would include 6 tags for running the matrix of tests that we currently have in travis, and one tag for circle.

This would also fix this issue: #1552.

WDYT @satra ?

[satra]

sounds good - we should also try to minimize the images. i'm looking into some of that - but may not happen immediately.

[oesteban]

yes, I also have that in mind. That's why I think we should split between a fully-fledged production image (that can be large because all the tools we want in it), but thin images for testing.

Currently I'm stuck with travis because of the size of the images.

[oesteban]

Images narrowed down from ~6GB to ~2GB

[oesteban]

I think this can be closed