Set `NODE_ENV` to support build with npm & Yarn. (mars#24)

Set `NODE_ENV` to support build yarn.

update to inner buildpack v3.0.0 "Preserve Source Maps"

Merge pull request mars#14 from mars/modular-runtime-env-vars

Modular runtime env vars

Make build scripts executable; fix for Dokku.

Node env vars

Merge pull request mars#6 from mars/react-app-env-vars

React app env vars

📚 doc general purpose SPA deployment (resolves mars#2)

Lock version of inner buildpack