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/commands/healthcheck.rb

@@ -11,14 +11,15 @@ class Mrsk::Commands::Healthcheck < Mrsk::Commands::Base
       "--label", "service=#{container_name}",
       "-e", "MRSK_CONTAINER_NAME=\"#{container_name}\"",
       *web.env_args,
+      *web.health_check_args,
       *config.volume_args,
       *web.option_args,
       config.absolute_image,
       web.cmd
   end
 
-  def curl
-    [ :curl, "--silent", "--output", "/dev/null", "--write-out", "'%{http_code}'", "--max-time", "2", health_url ]
+  def status
+    pipe container_id, xargs(docker(:inspect, "--format", DOCKER_HEALTH_STATUS_FORMAT))
   end
 
   def logs