DUB: Only fetch and cache packages in dub.json without running build

[Clinton]

Hi all,

I'm setting up a CircleCI config for my project. Right now I'm 
trying to cache dependencies before running builds. This way I 
can run "dub build --nodeps" immediately after the packages are 
cached to avoid extra network calls and speed it up.

I'm wondering if there's a way to isolate the part that downloads 
and caches all of the dependencies in dub.json without running 
the build.

Right now I have to run the build twice. First one, for the sake 
of downloading the dependencies and later for running if the 
cache exists. Having the build run the first time takes up a lot 
of time. I'm going for the "npm install" type of effect.

The reason the cache needs to be rebuilt each time is because 
CircleCI runs docker images. Once the deployment is finished, the 
image fs is destroyed so there's no way to hold onto the cache 
for future builds.