Add utilities for cacheable layers in docker builds

[solsson]

With CI builds or local round trips we want the following layers cached

• Production dependency changes
• Dev dependency changes
• Source changes (including source in monorepo deps)

Still WIP. We should port the tests from https://github.com/solsson/npm-monorepo-example to build-contracts here.

[solsson]

Ready for merge + publish, but how do we adopt the tests? Can we run them in build-contract's build-contract?

[solsson]

Built solsson/build-contract:build-cache-optimization@sha256:24f3f491df810a506aefe4d1a7c39731bf8c41f268c091b0a58e35479cee162c

[solsson]

@atamon You'll probably - unless we take a hard look at our .npmignores - bump into in-cluster builds having an issue with npm-ci.

npm-monorepo deps cause output like:

Unhandled rejection Error: Integrity check failed:
  Wanted: sha512-ZE1Y5ZFtMTTFek2fV+OzCCYkz7QGdAN4qocAF7BngFwbmppkhNODMrZJjL7k/V6J7GX7HGrA4dpOi57HqI6sNA==
   Found: sha512-eCIbNfUaEr+baeIZJhnrVarMvQ9izV5m4X5SkQXRQVbvINqzSywKM7YhnNVWzOpF1HBBitWGE4YxtGm3AC+DVg==
    at checksumError (/usr/local/lib/node_modules/npm/node_modules/cacache/lib/content/write.js:155:13)

A workaround is to monkeypatch package-lock.json. Not a big deal security-wise for monorepo deps, but annoying enough.

[solsson]

In-docker (not only in-cluster) build-contract also fails due to monorepo npm pack not executing the prepare script:

npm WARN lifecycle ...~prepare: cannot run in wd