Creating `dev` or `nightly` Docker images

[olliefr]

Currently the Docker images are not being built for releases tagged with a postfix, such as 0.0.2-beta.

This is a sane default setting, because you don't want your latest image (as well as your major and major.minor) to be updated when there is a beta release.

But at some point we may want to have the dev image, which is going to be built and pushed on any tag, including the postfixed ones.

This idea is parked here for now.