I do have a Docker container running a .net core 2 app.
The logging is configured using this code in Program.cs
public static IWebHostBuilde
I've been cursing this for a good while, and eventually found something that works for me.
First I enabled filesystem logging as per @dima_horror's answer.
Next I ran a command-line az webapp log tail --name myApp --resource-group myRg
That now seems to give me useful output (it gave me nothing prior to enabling filesystem logging).