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/main.rb

@@ -42,11 +42,6 @@ class Kamal::Cli::Main < Kamal::Cli::Base
         say "Ensure Traefik is running...", :magenta
         invoke "kamal:cli:traefik:boot", [], invoke_options
 
-        if KAMAL.config.role(KAMAL.config.primary_role).running_traefik?
-          say "Ensure app can pass healthcheck...", :magenta
-          invoke "kamal:cli:healthcheck:perform", [], invoke_options
-        end
-
         say "Detect stale containers...", :magenta
         invoke "kamal:cli:app:stale_containers", [], invoke_options.merge(stop: true)
 
@@ -77,9 +72,6 @@ class Kamal::Cli::Main < Kamal::Cli::Base
 
         run_hook "pre-deploy"
 
-        say "Ensure app can pass healthcheck...", :magenta
-        invoke "kamal:cli:healthcheck:perform", [], invoke_options
-
         say "Detect stale containers...", :magenta
         invoke "kamal:cli:app:stale_containers", [], invoke_options.merge(stop: true)
 
@@ -228,9 +220,6 @@ class Kamal::Cli::Main < Kamal::Cli::Base
   desc "env", "Manage environment files"
   subcommand "env", Kamal::Cli::Env
 
-  desc "healthcheck", "Healthcheck application"
-  subcommand "healthcheck", Kamal::Cli::Healthcheck
-
   desc "lock", "Manage the deploy lock"
   subcommand "lock", Kamal::Cli::Lock