Expand comment about `postinstall`

scripts/postinstall.js

-// There doesn't appear to be a way to skip `postinstall`,
-// as doing `yarn --ignore-scripts` could prevent native packages from building:
+// This allows `postinstall` to be skipped via `SKIP_YARN_POSTINSTALL=true`.
+// This is useful when building a Docker image because it lets you
+// take advantage of cached Docker layers and run Yarn
+// only if `package.json` or `yarn.lock` changed,
+// and not every time source code is updated:
+// https://nodejs.org/en/docs/guides/nodejs-docker-webapp/
+//
+// However, there doesn't appear to be a way to skip `postinstall`,
+// and doing `yarn --ignore-scripts` could prevent native packages from building:
 // https://github.com/yarnpkg/yarn/issues/4100
 
 const { execSync } = require("child_process");