If postinstall isn't run when a cache is available, how do I build my app without clearing cache?

Through extended investigation I’ve realized that the build step for my express.js app (using npm) will only run my postinstall script if I clear the node_modules cache.

If I do not do that, and my package-lock has not changed, digitalOcean’s app platform build process does not run npm install and therefore does not run the postinstall scripts either.

But if my build command that actually builds all my code is in a postinstall script, then of course my build doesn’t complete and the deploy fails when it goes to run the files that were never built.

I can make it work by manually clearing the cache every time, but what is the “right” way? How can I make digital ocean build my files on every deploy even if the package-lock is unchanged?