Draft: [soldier] steamrt-container: Add netbase
This adds /etc/services
to the runtime, required by Borderlands 2 as
reported in https://github.com/ValveSoftware/steam-runtime/issues/515
It might help other games too.
Addresses: https://gitlab.steamos.cloud/steamrt/tasks/-/issues/139
Draft because we still need a confirmation if it's okay to include netbase
in the runtime.