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.

test/integration/docker/deployer/app/Dockerfile

@@ -4,4 +4,5 @@ COPY default.conf /etc/nginx/conf.d/default.conf
 
 ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
+RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 

test/integration/docker/deployer/app/config/deploy.yml

@@ -8,6 +8,7 @@ env:
     CLEAR_TOKEN: '4321'
   secret:
     - SECRET_TOKEN
+asset_path: /usr/share/nginx/html/versions
 
 registry:
   server: registry:4443

test/integration/main_test.rb

@@ -20,6 +20,8 @@ class MainTest < IntegrationTest
     assert_app_is_up version: second_version
     assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "post-deploy"
 
+    assert_accumulated_assets first_version, second_version
+
     kamal :rollback, first_version
     assert_hooks_ran "pre-connect", "pre-deploy", "post-deploy"
     assert_app_is_up version: first_version
@@ -69,4 +71,10 @@ class MainTest < IntegrationTest
     def assert_no_remote_env_file
       assert_equal "nofile", docker_compose("exec vm1 stat /root/.kamal/env/roles/app-web.env 2> /dev/null || echo nofile", capture: true)
     end
+
+    def assert_accumulated_assets(*versions)
+      versions.each do |version|
+        assert_equal "200", Net::HTTP.get_response(URI.parse("http://localhost:12345/versions/#{version}")).code
+      end
+    end
 end