Remove the healthcheck step

To speed up deployments, we'll remove the healthcheck step.

This adds some risk to deployments for non-web roles - if they don't
have a Docker healthcheck configured then the only check we do is if
the container is running.

If there is a bad image we might see the container running before it
exits and deploy it. Previously the healthcheck step would have avoided
this by ensuring a web container could boot and serve traffic first.

To mitigate this, we'll add a deployment barrier. Until one of the
primary role containers passes its healthcheck, we'll keep the barrier
up and avoid stopping the containers on the non-primary roles.

It the primary role container fails its healthcheck, we'll close the
barrier and shut down the new containers on the waiting roles.

We also have a new integration test to check we correctly handle a
a broken image. This highlighted that SSHKit's default runner will
stop at the first error it encounters. We'll now have a custom runner
that waits for all threads to finish allowing them to clean up.

lib/kamal/cli/app.rb

@@ -14,9 +14,12 @@ class Kamal::Cli::App < Kamal::Cli::Base
             end
           end
 
+          barrier = Kamal::Cli::Healthcheck::Barrier.new if KAMAL.roles.many?
+
           on(KAMAL.hosts, **KAMAL.boot_strategy) do |host|
+            # Ensure primary role is booted first to allow the web barrier to be opened
             KAMAL.roles_on(host).each do |role|
-              Kamal::Cli::App::Boot.new(host, role, version, self).run
+              Kamal::Cli::App::Boot.new(host, role, self, version, barrier).run
             end
           end
 
@@ -284,4 +287,8 @@ class Kamal::Cli::App < Kamal::Cli::Base
     def version_or_latest
       options[:version] || KAMAL.config.latest_tag
     end
+
+    def web_and_non_web_roles?
+      KAMAL.roles.any?(&:running_traefik?) && !KAMAL.roles.all?(&:running_traefik?)
+    end
 end