This might require multiple steps.

The Vulkan libraries support was only added in libnvidia-container 1.0.3. As of this bug report, Funtoo provides libnvidia-container 1.0.2. Before that, the Vulkan environment was unavailable to the docker environment.

A quick investigation showed that despite the library being upgraded to 1.0.5, Steam does not find the Vulkan library, but vulkaninfo on a Vulkan enabled docker image works fine. I haven't investigated further.

Vulkan support is required for certain games and proton.