Building for multiple platforms

[Neils]

I maintain an open-source project written in D and I use DUB for 
building and my compiler backend is DMD. My dub.json file is 
rather simple: 
https://github.com/neilsf/XC-BASIC/blob/master/dub.json

I offer pre-built binaries for Linux x86, Linux x86_64, Windows 
and Mac OS.

I'm only doing this for a year so I am still quite a beginner in 
D and my workflow is the following when building the project:

1. Launch a VM using VirtualBox
2. dub build
3. Repeat for each platforms

The above is a painfully slow process. Is there any way to make 
it simpler and faster?

Any suggestions are warmly appreciated.