Investigate image size when using onbuild image

[fixe]

No description provided.

[promag]

Probably because of:

RUN apk --no-cache --virtual build-dependencies add g++ gcc git make python

which are kept.

[joaopaulofonseca]

@promag's got a point. These dependencies are adding around 154MB to the base image.

seegno/node         7        256.7 MB
seegno/node         7-test   256.7 MB
seegno/node         7-slim   256.7 MB
seegno/node         latest   256.7 MB
mhart/alpine-node   7        54.26 MB

What if we add those only if needed (build native dependencies, use dependencies directly from GitHub repo, etc, ..)?