Having the field set in modifyCommonNamespaceOptions is misleading,
since for the actual container it is later unconditionally overwritten
to point to the sandbox container.

So let's move its setting to modifyHostOptionsForSandbox (renamed from
modifyHostNetworkOptionForSandbox as it's not about network only), since
that reflects what actually happens in practice.

This commit is purely a refactor, it doesn't change any behavior.