Asset paths

During deployments both the old and new containers will be active for a
small period of time. There also may be lagging requests for older CSS
and JS after the deployment.

This can lead to 404s if a request for old assets hits a new container
or visa-versa.

This PR makes sure that both sets of assets are available throughout the
deployment from before the new version of the app is booted.

This can be configured by setting the asset path:

```yaml
asset_path: "/rails/public/assets"
```

The process is:
1. We extract the assets out of the container, with docker run, docker
cp, docker stop. Docker run sets the container command to "sleep" so
this needs to be available in the container.
2. We create an asset volume directory on the host for the new version
of the app on the host and copy the assets in there.
3. If there is a previous deployment we also copy the new assets into
its asset volume and copy the older assets into the new asset volume.
4. We start the new container mapping the asset volume over the top of
the container's asset path.

This means the both the old and new versions have replaced the asset
path with a volume containing both sets of assets and should be able
to serve any request during the deployment. The older assets will
continue to be available until the next deployment.

lib/kamal/configuration/role.rb

@@ -48,11 +48,15 @@ class Kamal::Configuration::Role
     argumentize "--env-file", host_env_file_path
   end
 
+  def asset_volume_args
+    asset_volume&.docker_args
+  end
+
   def health_check_args(cord: true)
     if health_check_cmd.present?
       if cord && uses_cord?
         optionize({ "health-cmd" => health_check_cmd_with_cord, "health-interval" => health_check_interval })
-          .concat(["--volume", "#{cord_host_directory}:#{cord_container_directory}"])
+          .concat(cord_volume.docker_args)
       else
         optionize({ "health-cmd" => health_check_cmd, "health-interval" => health_check_interval })
       end
@@ -74,15 +78,23 @@ class Kamal::Configuration::Role
   end
 
   def uses_cord?
-    running_traefik? && cord_container_directory.present? && health_check_cmd.present?
+    running_traefik? && cord_volume && health_check_cmd.present?
   end
 
   def cord_host_directory
-    File.join config.run_directory_as_docker_volume, "cords", [full_name, config.run_id].join("-")
+    File.join config.run_directory_as_docker_volume, "cords", [container_prefix, config.run_id].join("-")
+  end
+
+  def cord_volume
+    if (cord = health_check_options["cord"])
+      @cord_volume ||= Kamal::Configuration::Volume.new \
+        host_path: File.join(config.run_directory, "cords", [container_prefix, config.run_id].join("-")),
+        container_path: cord
+    end
   end
 
   def cord_host_file
-    File.join cord_host_directory, CORD_FILE
+    File.join cord_volume.host_path, CORD_FILE
   end
 
   def cord_container_directory
@@ -90,7 +102,7 @@ class Kamal::Configuration::Role
   end
 
   def cord_container_file
-    File.join cord_container_directory, CORD_FILE
+    File.join cord_volume.container_path, CORD_FILE
   end
 
 
@@ -110,10 +122,37 @@ class Kamal::Configuration::Role
     name.web? || specializations["traefik"]
   end
 
-  def full_name
+  def container_name(version = nil)
+    [ container_prefix, version || config.version ].compact.join("-")
+  end
+
+  def container_prefix
     [ config.service, name, config.destination ].compact.join("-")
   end
 
+  def asset_path
+    specializations["asset_path"] || config.asset_path
+  end
+
+  def assets?
+    asset_path.present? && running_traefik?
+  end
+
+  def asset_volume(version = nil)
+    if assets?
+      Kamal::Configuration::Volume.new \
+        host_path: asset_volume_path(version), container_path: asset_path
+    end
+  end
+
+  def asset_extracted_path(version = nil)
+    File.join config.run_directory, "assets", "extracted", container_name(version)
+  end
+
+  def asset_volume_path(version = nil)
+    File.join config.run_directory, "assets", "volumes", container_name(version)
+  end
+
   private
     attr_accessor :config