Add public_origin attribute to Configuration::DevServerSettings

This is an _optional_ configuration attribute which will be used to
generate the URLs in asset tags. If this isn't set, the asset origin
defaults to `"http://#{dev_server.host}:#{dev_server.port}"`, so
`public_origin` only needs to be set if that default doesn't work. In
essence, this setting allows different origins to be used when the Rails
backend connects to the dev server to get the manifest and when the
browser requests assets from the dev server. (Often, though, the same
origin can be used, which is why this setting is optional.)

Author: rmacklin

Diff for: README.md

@@ -96,13 +96,23 @@ config.external_asset_pipeline.dev_server.host = 'localhost'
 config.external_asset_pipeline.dev_server.port = 9000
 ```
 
+You may _optionally_ also configure the `dev_server.public_origin` setting,
+which will be used to generate the URLs in your asset tags. If this isn't set,
+the asset origin defaults to `"http://#{dev_server.host}:#{dev_server.port}"`,
+so `public_origin` only needs to be set if that default doesn't work <sup>1</sup>.
+
 If the dev server is enabled but not running (i.e. we can't establish a
 connection to that port), the app will automatically fall back to returning
 assets from disk.
 
 See an example in
 [`examples/demo_app/config/environments/development.rb`](./examples/demo_app/config/environments/development.rb)
 
+<sup><sup>1</sup> For example, if you're running everything in docker containers
+then the origin from which the browser requests assets may be different than the
+origin that the rails container uses to request the manifest from the dev server
+container.</sup>
+
 [`webpack-dev-server`]: https://github.com/webpack/webpack-dev-server
 
 ## Development

Diff for: lib/external_asset_pipeline/configuration.rb

@@ -19,7 +19,8 @@ class DevServerSettings
       attr_accessor :connect_timeout,
                     :enabled,
                     :host,
-                    :port
+                    :port,
+                    :public_origin
 
       def initialize
         @connect_timeout = 0.01

Diff for: lib/external_asset_pipeline/dev_server.rb

@@ -13,7 +13,7 @@ def get(path)
     end
 
     def origin
-      "http://#{@config.host}:#{@config.port}"
+      @config.public_origin || "http://#{@config.host}:#{@config.port}"
     end
 
     def running?

Diff for: test/external_asset_pipeline/configuration_test.rb

@@ -45,18 +45,21 @@ def test_dev_server_settings
       assert_nil config.dev_server.enabled
       assert_nil config.dev_server.host
       assert_nil config.dev_server.port
+      assert_nil config.dev_server.public_origin
 
       config.configure do |c|
         c.dev_server.connect_timeout = 0.5
         c.dev_server.enabled = true
         c.dev_server.host = 'localhost'
         c.dev_server.port = 9000
+        c.dev_server.public_origin = 'http://example-app.test'
       end
 
       assert_equal 0.5, config.dev_server.connect_timeout
       assert_equal true, config.dev_server.enabled
       assert_equal 'localhost', config.dev_server.host
       assert_equal 9000, config.dev_server.port
+      assert_equal 'http://example-app.test', config.dev_server.public_origin
     end
 
     def test_logger

Diff for: test/external_asset_pipeline/dev_server_test.rb

@@ -35,6 +35,14 @@ def test_origin
       dev_server = DevServer.new(config)
 
       assert_equal "http://#{config.host}:9666", dev_server.origin
+
+      config.public_origin = 'http://myapp.test:4444'
+
+      assert_equal 'http://myapp.test:4444', dev_server.origin
+
+      config.public_origin = nil
+
+      assert_equal "http://#{config.host}:9666", dev_server.origin
     end
 
     def test_running