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/utils/healthcheck_poller.rb

@@ -0,0 +1,39 @@
+class Mrsk::Utils::HealthcheckPoller
+  TRAEFIK_HEALTHY_DELAY = 1
+
+  class HealthcheckError < StandardError; end
+
+  class << self
+    def wait_for_healthy(pause_after_ready: false, &block)
+      attempt = 1
+      max_attempts = MRSK.config.healthcheck["max_attempts"]
+
+      begin
+        case status = block.call
+        when "healthy"
+          sleep TRAEFIK_HEALTHY_DELAY if pause_after_ready
+        when "running" # No health check configured
+          sleep MRSK.config.readiness_delay if pause_after_ready
+        else
+          raise HealthcheckError, "container not ready (#{status})"
+        end
+      rescue HealthcheckError => e
+        if attempt <= max_attempts
+          info "#{e.message}, retrying in #{attempt}s (attempt #{attempt}/#{max_attempts})..."
+          sleep attempt
+          attempt += 1
+          retry
+        else
+          raise
+        end
+      end
+
+      info "Container is healthy!"
+    end
+
+    private
+      def info(message)
+        SSHKit.config.output.info(message)
+      end
+  end
+end