Remove volumes after build

Since these volumes are both unreferencable, and should be completely
reproducible by subsequent builds, they can be safely removed.

Built on #7672
Resolves #6993 which was closed but not dealt with.
Technically yes, we could do that, however this would complicate things in other places.
And being that we are specifically dealing with volumes, I'd like to make sure we are using the volume struct rather than just some string.