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

[Marc]

> 
> 	I think this will copy duplicates, duplicates increase still layer
> size so you have 2x size of a default /usr
> 
> 	so you can only copy individual files
> 
> You are right, extra files appear in the diff! I was thinking that the
> files already present would be discarded. Copying individual files in
> this case is not the solution: there are plenty of files (305 if counting
> includes)
>
> So this is a bit more complicated to achieve something cleaner. Maybe
> install the binaries in an alternate directory in the build step?

What about compiling with prefix /usr/local/ I think this is empty in alpline. I have the impression more vendors do this.



> I made a new Dockerfile based on this idea, now the result is 66MB. 

Nice!!!