Currently the primary on-ramp for discovery of container tabs (I know this won't be final - just capturing how it is today) is quite limited - you have to really know it's there and go looking for it.
I suspect most users will discover it via right clicking a link and seeing Open Link in Container >
Alternatively, right clicking a tab might lead to the user accidentally discovering "Reopen in container".
When I first heard about the feature and was looking to find how to use Container Tabs, the first place I went to was the page settings screen:
This seems like a possibly sensible place to expose the "always open this site in X container" feature, to avoid users needing to manually type a domain into the settings panel. It could be a dropdown here, like the other page settings, with the container tab colour showing, and the dropdown list giving you the available containers?
