Fetch and pre-build dub dependencies

[Tamas]

On Wednesday, 7 March 2018 at 04:28:11 UTC, Seb wrote:
> For run.dlang.io, I fetch all dependencies and build them into 
> the Docker image. That has the advantage that executions with 
> dub packages are a lot faster.
> It's just dub fetch and dub build though. Take a look:
>
> https://github.com/dlang-tour/core-exec/blob/master/Dockerfile

Thank you for showing your solution! Using dub --single is neat 
and works well for this!