'push' sub-command for the 'registry' command

[kcq]

[shogohida]

Hi @kcq, is this issue still relevant? It seems 'push' sub-command already exists. I would like to contribute to this if anyone still needs to fix it.

[kcq]

@shogohida there's a placeholder for the 'push' subcommand, but there's no actual 'business logic' for it. The push handler (OnPushCommand: https://github.com/slimtoolkit/slim/blob/master/pkg/app/master/commands/registry/handler.go#L146 ) just prints that it's done and that's it :-)