[keyserver] Cache Webpack builds in Docker build

Summary:
I'm iterating on files in the keyserver directory, and every time I rebuild the Docker image the Webpack builds get re-run, even though none of the files they depend on have changed.

This diff makes sure that the Webpack builds only get rerun if one of the files they depend on changes.

Depends on D4174

Test Plan: I observed that the Webpack builds stopped appearing on every Docker build after this diff

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4175