IHostBuilder builder = new HostBuilder()
    .ConfigureServices((hostBuilderContext, services) => {
        services.AddHostedService<MyService>();
     });

public class MyService : IHostedService {
    public async Task StartAsync(CancellationToken cancellationToken) { }
    public async Task StopAsync(CancellationToken cancellationToken) { }
}

public class MyCustomProvider : IServiceProvider {

    SomeContainer _someContainer;

    public MyCustomProvider(SomeContainer someContainer) {
        _container = container;
    }
    public object GetService(Type serviceType) {
        return _container.Get(serviceType);
        // This is only called once, and serviceType is typeof(IHost)
    }
}

IHostBuilder builder = new HostBuilder()
    .UseServiceProviderFactory(new MyContainerFactory())
    .ConfigureServices((hostBuilderContext, services) => {
        services.AddHostedService<MyService>();
     });

class MyContainerFactory : IServiceProviderFactory<SomeContainer> {
    public SomeContainer CreateBuilder(IServiceCollection services) {
        return SomeContainer.SetupStuff().Create();
    }
    public IServiceProvider CreateServiceProvider(SomeContainer container) {
        return new MyCustomProvider(container);
    }
}