The main advantage in my use case is in fact isolation (network and volumes) and a well defined API enabling management of those containers in production (not k8s, a tiny subnet of that perhaps).

The isolation could be achieved using namespaces directly. But the API, tooling and registry add a lot of value that would otherwise require a lot of development.

Also last time I looked hypervisors aren't possible on all cloud vendors, unless you have a bare metal server. This matters in my case. Maybe it has changed in the past 3 years.

When docker fits it's great. Same can be said of k8s, where there are a whole bunch of additional benefits.

Swings and roundabouts.