Pass KAMAL_VERSION env var to container run

In lieu of a general purpose mechanism to pass dynamically-evaluated env-vars at container execution time, we can pass the `config.version` as KAMAL_VERSION to avoid having to take apart the container name just to determine the SHA of the deployed version in the entrypoint.

lib/kamal/commands/app.rb

@@ -18,6 +18,7 @@ def run(hostname: nil)
       "--name", container_name,
       *(["--hostname", hostname] if hostname),
       "-e", "KAMAL_CONTAINER_NAME=\"#{container_name}\"",
+      "-e", "KAMAL_VERSION=\"#{config.version}\"",
       *role_config.env_args,
       *role_config.health_check_args,
       *config.logging_args,