Use the smallest image that satisfies your requirements. For Linux, you could use a runtime-alpine .NET Core image, which has been supported since the release of .NET Core 2.1. For Windows, if you are using the full .NET Framework, then you need to use a Windows Server Core image (runtime-only image, such as 4.7.2-windowsservercore-ltsc2016). Runtime-only images are smaller but do not support workloads that require the .NET SDK.
Note
ACI cannot pull images from non OCI-compliant registries.