Make script/ entrypoints POSIX sh; add bootstrap and setup

README.md

@@ -107,9 +107,13 @@ needed in your projects.
 This repository adheres to the
 [Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
 standard: normalized scripts in `script/` are the entrypoints for the
-development workflow, and the Makefile targets are thin shims that call them. We
-provide:
+development workflow, and the Makefile targets are thin shims that call them.
+The scripts are POSIX sh (not bash) so they run in minimal containers such as
+alpine. We provide:
 
+- `script/bootstrap` — install all dependencies (yarn install)
+- `script/setup` — set up the repo for development after a fresh clone: runs
+  `script/bootstrap`, then `script/install-precommit`
 - `script/projectname` — output the project name (our own extension); used by
   `script/docker` for the image tag
 - `script/test` — run the test suite (no tests defined here)