Add container name env var for containers

Because the container name is generated it isn't possible to
determine this inside the container.

This adds the MRSK_CONTAINER_NAME env var when running the
container so it can be read by the service running inside the
container.

lib/mrsk/commands/healthcheck.rb

@@ -9,6 +9,7 @@ class Mrsk::Commands::Healthcheck < Mrsk::Commands::Base
       "--name", container_name_with_version,
       "--publish", "#{EXPOSED_PORT}:#{config.healthcheck["port"]}",
       "--label", "service=#{container_name}",
+      "-e", "MRSK_CONTAINER_NAME=\"#{container_name}\"",
       *web.env_args,
       *config.volume_args,
       config.absolute_image,