The Containers SIG is fairly broad. People who want to talk about application containers and also work on development with us might find it easier to discuss on Discourse and then be asked to file an issue on Pagure rather than having to go full on IRC/ML/Pagure. Pagure is already a learning curve since it's unfamiliar to everyone who isn't familiar with Fedora.
This SIG isn't heavy on kernel and OS development and therefore might be a good entry point for beginners as well. Asking people to go to mailing lists and IRC might not be the best way to have some fresh breeze community engagement. A Containers SIG is a great way to gather developers' interest outside of the usual Fedora world which hopefully leads to some interest in Fedora as well, especially since they'd already be on the forums and be able to explore other stuff they might be interested in.