Create a stable tag on dockerhub

[gwleclerc]

Now that Smocker has reached 1K stars, I'm not convinced that everyone uses fixed versions of Smocker images. I like the fact that the latest is updated with each release and I'd like to keep it that way, but if a version introduces bugs it could have an impact on those using the latest. So it might be a good idea to introduce a stable version that would only change once the latest has been tested over time.