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

[Marc]

I can definitely remember having a performance difference between my container and a vm. I never bothered to research it any further and thought maybe it was related to older cgroups implementation, oc, or older distro. 

> 
> By any chance have you measured the performance difference between GNU
> libc and MUSL?
> 
> 
> Best Regards,
> Taavi
>