Work out why InheritDoc is failing in Docker

[Jon Skeet]

It works locally... but in Docker we have things like this:

obj/Release/netcoreapp3.1/NodaTime.Serialization.SystemTextJson.xml(38,10): InheritDocTask warning IDT002: No matching documentation could be found for: M:NodaTime.Serialization.SystemTextJson.DelegatingConverterBase1.Write(System.Text.Json.Utf8JsonWriter,0,System.Text.Json.JsonSerializerOptions), which attempts to inherit from: M:System.Text.Json.Serialization.JsonConverter1.Write(System.Text.Json.Utf8JsonWriter,0,System.Text.Json.JsonSerializerOptions) [/source/src/NodaTime.Serialization.SystemTextJson/NodaTime.Serialization.SystemTextJson.csproj::TargetFramework=netcoreapp3.1]