Add option to install local build

In the process of supporting Rails 8 via #1213, we realized we needed to
install a local build.

This commit simply introduces a `--suspenders-local` option, and
documents how to build locally.

Co-Authored-By: Nick Charlton <[email protected]>

CONTRIBUTING.md

@@ -46,6 +46,32 @@ This is a time for discussion and improvements,
 and making the necessary changes will be required before we can
 merge the contribution.
 
+## Testing a Local Build
+
+There are cases where you'll want to use a local build, rather tha a released
+version.
+
+```
+SUSPENDERS_LOCAL_PATH=/path/to/suspenders \
+ rails new app_name \
+ --suspenders-local \
+ --skip-rubocop \
+ --skip-test \
+ -d=postgresql \
+ -m=https://raw.githubusercontent.com/thoughtbot/suspenders/main/lib/install/web.rb
+```
+
+You can also change the path used for the application template by setting the
+`-m` flag to something else.
+
+```
+rails new app_name \
+ --skip-rubocop \
+ --skip-test \
+ -d=postgresql \
+ -m=path/to/suspenders/lib/install/web.rb
+```
+
 ## Testing Generators
 
 There is a smaller dummy application at `test/dummy`. This application is used

lib/install/web.rb

@@ -27,6 +27,8 @@ def apply_template!
       gem_group :development, :test do
         if ARGV.include?("--suspenders-main")
           gem "suspenders", github: "thoughtbot/suspenders", branch: "main"
+        elsif ARGV.include?("--suspenders-local") && ENV["SUSPENDERS_LOCAL_PATH"]
+          gem "suspenders", path: ENV.fetch("SUSPENDERS_LOCAL_PATH")
         else
           gem "suspenders"
         end