I have Signal and microG with push notifications. Signal still uses websocket on my device. So, I guess it would be fine without microG push.

Docker is not only about dependency management. It also offers service “composing”, via docker compose, and network isolation for each service.

Although I personally love Nix, and I run NixOS on some of my servers, I do not believe it can replace Docker/Podman. Unless you go the NixOS Containers route.