Improve docker intermediate images

[jkliff]

• reduces RUN statements and, after apt-get usages, cleanup to reduce size of said intermediate images;
• add targets to Makefile to conveniently export images (when for example not running on same host where was built)

[lava]

This looks very nice, but why are you removing /var/lib/apt/lists/* in every Makefile? Shouldn't it be enough to do it in mail-base?

[jkliff]

If not removed it will be included in the overlays increasing needlessly their size. Arguably, the tradeoff is that one has to apt-get update again at each Dockerfile, which might increase the build time; but overlays might need to be stored, published, shared, etc, so keeping their size as small as possible is considered a good practice (see section apt-get on [1]).

[1] https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/

[lava]

Ok then, thanks for contributing!