My app running in a docker container on Azure Webapps for Containers tries to access a connection string through an environment variable. I've added it to the Application Settings in the Azure UI but I can't access it through my code, specifically my ASP.NET Core application is returning null.
I know that the logs won't show it being added as a -e connstring=myconnstring argument in the docker run command, but it should never the less be present in the container.
It turns out, by using the Advanced Tools -> Environment Kudu service in Azure, the connection string environment variable names were being prefixed with SQLAZURECONNSTR_.
I know it is a convention to have these kind of prefixes on environment variables when reading them with the .NET Core environment variable configuration provider as described here, but quite why Azure adds these prefixes automatically, apparently without documenting this behaviour anywhere, is unclear to me.