Cabal: download dependencies for later offline build

Dear reader, I'm experimenting with creating my own little build server using disk images as job input/output and KVM to run them[1]. As a security measure, the build process does not have any network access during the build, so I need to make sure everything is available before the build starts. What is the easiest way to "take a cabal build offline" as it were, so I can make sure a cabal build will work without a network connection? Greetings, Bram [1] https://github.com/bneijt/construir/