If you're running multiple containers of images that are themselves derived from a same image, is it still a one-time cost? Is Docker smart enough to run "only one" instance of Ubuntu, for example?
What resources are you concerned about being consumed?
- There will only ever be one running kernel with docker.
- The base filesystem layer, if identically hashed, will be shared as an overlay filesystem.
- The memory footprint of whatever each container runs (which will generally not be a full from runlevel0 system) will not be shared, except in the sense that binaries loaded into ram from the same overlay filesystem will have some of their disk pages shared.
What stormbrew said above. It's probably not an absolutely O(1) cost but in practice I haven't noticed the difference. I definitely have noticed a saving from pushing many different "slim" alpine images around (alpine is not at fault here, but the more differences your images have, the less you get to re-use existing layers).
