You are buying hosting - an amount of memory, CPU, disk and bandwidth. Apps are what you run inside it.
That means there is no separate "Docker product" to buy. Your plan says how much memory and CPU your account has, and every app you install runs inside that. Three apps on a 4 GB plan share 4 GB between them; they do not get 4 GB each.
Two numbers on your plan matter for apps:
- Memory and CPU - the total for everything in your account.
- App limit - the most apps you may have installed at once.
In practice memory is the real limit. A small app might need 128 MB, a large one a gigabyte or more, so how many you can run at the same time depends on which ones you pick.