Move health checks into Docker

Replaces our current host-based HTTP healthchecks with Docker
healthchecks, and adds a new `healthcheck.cmd` config option that can be
used to define a custom health check command. Also removes Traefik's
healthchecks, since they are no longer necessary.

When deploying a container that has a healthcheck defined, we wait for
it to report a healthy status before stopping the old container that it
replaces. Containers that don't have a healthcheck defined continue to
wait for `MRSK.config.readiness_delay`.

There are some pros and cons to using Docker healthchecks rather than
checking from the host. The main advantages are:

- Supports non-HTTP checks, and app-specific check scripts provided by a
  container.
- When booting a container, allows MRSK to wait for a container to be
  healthy before shutting down the old container it replaces. This
  should be safer than relying on a timeout.
- Containers with healthchecks won't be active in Traefik until they
  reach a healthy state, which prevents any traffic from being routed to
  them before they are ready.

The main _disadvantage_ is that containers are now required to provide
some way to check their health. Our default check assumes that `curl` is
available in the container which, while common, won't always be the
case.

lib/mrsk/configuration/role.rb

@@ -35,6 +35,21 @@ class Mrsk::Configuration::Role
     argumentize_env_with_secrets env
   end
 
+  def health_check_args
+    if health_check_cmd.present?
+      optionize({ "health-cmd" => health_check_cmd, "health-interval" => "1s" })
+    else
+      []
+    end
+  end
+
+  def health_check_cmd
+    options = specializations["healthcheck"] || {}
+    options = config.healthcheck.merge(options) if running_traefik?
+
+    options["cmd"] || http_health_check(port: options["port"], path: options["path"])
+  end
+
   def cmd
     specializations["cmd"]
   end
@@ -75,8 +90,6 @@ class Mrsk::Configuration::Role
       if running_traefik?
         {
           "traefik.http.routers.#{traefik_service}.rule" => "PathPrefix(`/`)",
-          "traefik.http.services.#{traefik_service}.loadbalancer.healthcheck.path" => config.healthcheck["path"],
-          "traefik.http.services.#{traefik_service}.loadbalancer.healthcheck.interval" => "1s",
           "traefik.http.middlewares.#{traefik_service}-retry.retry.attempts" => "5",
           "traefik.http.middlewares.#{traefik_service}-retry.retry.initialinterval" => "500ms",
           "traefik.http.routers.#{traefik_service}.middlewares" => "#{traefik_service}-retry@docker"
@@ -125,4 +138,8 @@ class Mrsk::Configuration::Role
         new_env["clear"] = (clear_app_env + clear_role_env).uniq
       end
     end
+
+    def http_health_check(port:, path:)
+      "curl -f #{URI.join("http://localhost:#{port}", path)} || exit 1" if path.present? || port.present?
+    end
 end