[keyserver] Avoid redownloading Node from nvm whenever any files change

Summary:
Since we don't download Node until the first time source-nvm.sh is run, that means Node gets downloaded in the prod-build step.

prod-build needs to rerun whenever any files change, but Node should only need to get downloaded when the version changes.

This diff uses Docker build caching to call source-nvm.sh earlier, so that we can make it cheaper to rebuild when a source file changes.

Depends on D4213

Test Plan: Change a file in keyserver/src and make sure that nvm doesn't re-download Node when running the Docker build again

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4214