Updated Docker images (9.18, 9.20, 9.21) - now based on Alpine Linux

[Marc]

> 
> What’s the size difference for you?
> 
> I mean if someone wants to play with our Dockerfile and there’s a
> significant reduction is size, I would be convinced. But in a world,
> where a mobile application that does absolutely nothing has 4 GB, I feel
> like 130 MB is on the low side of the scale.
> 

I don't look at it like that. I see half back up space required, half of the wear level on ssds, half of the power consumption, twice as fast to deploy. And if you have a lot of downloads this starts counting.

I have been telling Microsoft and Bill Gates to reduce updates and footprints, but I don't think they care to much about the environment ;)