Format shell scripts using shfmt

Shell scripts are now formatted using shfmt:
https://github.com/mvdan/sh/blob/master/cmd/shfmt/shfmt.1.scd

This is enforced in CI, and formatting can be applied locally
using a new `make format` target.

Notably, this also reformats the scripts to use tabs instead of spaces,
since tabs are sadly the only way to be able to properly indent bash
here documents (which is something the scripts here will be using a
lot of soon) without having to resort to mixed tabs and spaces
indentation in the same file. (Plus tabs is also the shfmt default style.)

GUS-W-16808198.

.editorconfig

@@ -0,0 +1,26 @@
+# https://editorconfig.org
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.sh]
+# We have to use tabs in shell scripts otherwise we can't indent here documents:
+# https://www.gnu.org/software/bash/manual/html_node/Redirections.html#Here-Documents
+indent_style = tab
+shell_variant = bash
+
+# Catches scripts that we can't give a .sh file extension, such as the Buildpack API scripts.
+[**/bin/**]
+indent_style = tab
+shell_variant = bash
+
+[.hatchet/repos/**]
+ignore = true
+
+[Makefile]
+indent_style = tab

.github/workflows/ci.yml

@@ -10,6 +10,10 @@ on:
 permissions:
   contents: read
 
+env:
+  # Used by shfmt and more.
+  FORCE_COLOR: 1
+
 jobs:
   lint:
     runs-on: ubuntu-24.04
@@ -23,6 +27,10 @@ jobs:
           ruby-version: "3.3"
       - name: Run ShellCheck
         run: make lint-scripts
+      - name: Run shfmt
+        uses: docker://mvdan/shfmt:latest
+        with:
+          args: "--diff ."
       - name: Run Rubocop
         run: bundle exec rubocop
 

Makefile

@@ -1,20 +1,26 @@
 # These targets are not files
-.PHONY: lint lint-scripts lint-ruby run publish
+.PHONY: lint lint-scripts lint-ruby check-format format run publish
 
 STACK ?= heroku-24
 FIXTURE ?= spec/fixtures/python_version_unspecified
 
 # Converts a stack name of `heroku-NN` to its build Docker image tag of `heroku/heroku:NN-build`.
 STACK_IMAGE_TAG := heroku/$(subst -,:,$(STACK))-build
 
-lint: lint-scripts lint-ruby
+lint: lint-scripts check-format lint-ruby
 
 lint-scripts:
 	@git ls-files -z --cached --others --exclude-standard 'bin/*' '*/bin/*' '*.sh' | xargs -0 shellcheck --check-sourced --color=always
 
 lint-ruby:
 	@bundle exec rubocop
 
+check-format:
+	@shfmt --diff .
+
+format:
+	@shfmt --write --list .
+
 run:
 	@echo "Running buildpack using: STACK=$(STACK) FIXTURE=$(FIXTURE)"
 	@docker run --rm -it -v $(PWD):/src:ro --tmpfs /app -e "HOME=/app" -e "STACK=$(STACK)" "$(STACK_IMAGE_TAG)" \