Add a dev or nightly docker image

[rgaudin]

Currently, we only offer docker images for released version. Having up-to-date docker images is generally helpful and nice but in our situation of not releasing for 6months+, it's become much needed ; particularly since we have other tools integrating zimcheck (zimfarm, scraper CI).

In #249, I proposed to create a nightly docker image tag that would be built exactly as the release one, but instead of downloading a release tarball, it would donwload the latest nightly one. This is a close-to-zero effort solution that would offer a 24h-old docker image to all.

@kelson42 is in favor of a standard dev image that would be built from the master source. While I agree it has additional advantages (zero delay, tests building as well), it requires more effort: we need to create that Dockerfile from scratch and maintain it over time while kiwix-build already constructs and publish builds for the nightlies.

[kelson42]

Difficult ticket, an other approach would be to just acknowledge our failure of releasing zim-tools in a reasonable delay, wait this is done and suffer... and do our best that this don't occurr any time soon again?

[rgaudin]

I don't get how this would be better than the nightly way. There must be a downside of publishing this that I don't see. Obviously, as soon as we have a release, our integrations would use the released version.

[kelson42]

The only downside I see is that this would be a premiere with Docker and that I think this is confusing with the dev tag we already use everywhere... But it is quite clear to me as well that I'm overvaluated this problem/argument.

[mgautierfr]

Another solution would be to not wait form python-libzim and release libzim the sooner possible :)