Share the package download cache across builds

[jackfirth]

Multiple images for each installer are built, and each image needs packages downloaded and installed in order to run tests. Downloading packages takes a significant amount of build time that scales linearly with the number of Racket environments to test, which itself is a combinatorical explosion (minimal vs full, x64 vs natipkg vs i386, source build vs installer script, etc.). Caching these downloads would go a long way towards speeding up the build.

Packages are needed when images are built, not when containers are run. This means a docker volume can't be used for the build cache because builds aren't meant to use volumes. Instead, the entire pre-release catalog should be archived and stored in its own docker image. Then, the multi-stage build can extract the downloaded archive from the catalog archival image and use that archive as a directory package catalog.