Explain Docker secrets handling

[nbarbettini]

Got cut from 1.0: the deployment chapter should explain how to handle secrets in a Docker environment. Right now, it tells you to disable Facebook login if you're on Docker (which isn't ideal).

[GarethOates]

I've just ran through all of your book just now and I've enjoyed it very much. Thank you for taking the time to create such a well structured and well paced instruction guide.

Am I right in thinking that one way to pass the app Id and secret into the container is to set the ID and Secret as environmental variables when you run the build command, and in the Dockerfile run the dotnet user-secrets set Facebook:AppId ENV_VAR1 like that?

Is this an acceptable approach?