Cache Webpacker builds between deploys

 ## Goal
Facilitate Webpacker's existing caching to skip running Webpack on
builds where no changes are made.

 ## Approach
Cache Webpacker's default pack output and cache paths between deploys.

 ## Context
Currently all deploys using Webpacker will recompile on every deploy
even if there are no files changed. This is because the digest file and
packs directory is not cached between deploys.

By default, Webpacker stores a cache digest in `tmp/cache/webpacker` and
its packs in `public/packs`. Webpacker iterates over the watched paths to
compute a digest based on the mtime of the files. If none of the files
have changed and the packs directory is in place, running Webpack is
skipped, saving a lot of time (e.g. my company's Rails app takes 8
minutes to run webpack).

 ## Caveats
The environment variable `CI` needs to be set as the mtime does not
seem to be preserved correctly across deploys. If `ENV['CI']` is set,
Webpacker will use the file contents to compute the cache digest file
instead of the files' mtime.

I'm not familiar enough with the Heroku infrastructure to understand
why the mtime isn't being preserved. I did notice that `cp`ing is done
via `-a` which preserves timestamps via `--preserve=all` but that
doesn't seem to be enough.

 ## Further Notes
This could technically be solved without modifying either Webpacker or
the Heroku build pack but I thought this would be the best place as it
fixes all existing configurations provided they used the default
settings. This would also make the default "just work" instead of
requiring end users to configure their Rails apps specifically for
Heroku.

I didn't mean for this pull request to be the canonical place to fix
this issue but merely an issue report with a solution attached.

 ## Related
* rails/webpacker#1347

Signed-off-by: Andrew Kane <[email protected]>

lib/language_pack/rails4.rb

@@ -66,6 +66,14 @@ def default_assets_cache
     "tmp/cache/assets"
   end
 
+  def public_packs_folder
+    "public/packs"
+  end
+
+  def default_webpacker_cache
+    "tmp/cache/webpacker"
+  end
+
   def cleanup
     super
     return if assets_compile_enabled?
@@ -87,7 +95,9 @@ def run_assets_precompile_rake_task
         topic("Preparing app for Rails asset pipeline")
 
         @cache.load_without_overwrite public_assets_folder
+        @cache.load_without_overwrite public_packs_folder
         @cache.load default_assets_cache
+        @cache.load default_webpacker_cache
 
         precompile.invoke(env: rake_env)
 
@@ -100,7 +110,9 @@ def run_assets_precompile_rake_task
 
           cleanup_assets_cache
           @cache.store public_assets_folder
+          @cache.store public_packs_folder
           @cache.store default_assets_cache
+          @cache.store default_webpacker_cache
         else
           precompile_fail(precompile.output)
         end