runtime: Make bumblebee.socket visible in container
When using Primus/Bumblebee, having this available is apparently enough for at least some games to work, and having this not available will result in the driver failing to load.
Note that Primus/Bumblebee is not really supported by any of the GPU or driver vendors. Using PRIME render offloading (DRI_PRIME=1 for open-source Mesa drivers, __NV_PRIME_RENDER_OFFLOAD=1 for the proprietary NVIDIA drivers) is likely to work more reliably.
Helps: https://github.com/ValveSoftware/steam-runtime/issues/372