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/base.rb

@@ -2,6 +2,8 @@ module Mrsk::Commands
   class Base
     delegate :sensitive, :argumentize, to: Mrsk::Utils
 
+    DOCKER_HEALTH_STATUS_FORMAT = "'{{if .State.Health}}{{.State.Health.Status}}{{else}}{{.State.Status}}{{end}}'"
+
     attr_accessor :config
 
     def initialize(config)