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Docker and Configuration on Azure Web App for Containers: Whither Colons?

App Services have long been a super simple way to spin up a web app in Azure. The barrier to entry is low, maintenance is easy. It just works. App Services recently got a turbo boost in the form of Azure App Service on Linux. Being able to deploy to Linux is exciting enough; but the real reason this is notable because you can deploy Docker images that will be run as app services.

I cannot over-emphasise just how easy this makes getting a Docker image into Production. Yay Azure!

The Mystery of Configuration#

Applications need configuration. ASP.Net Core applications are typically configured by an appsettings.json file which might look like so:

{
"Parent": {
"ChildOne": "I'm a little teapot",
"ChildTwo": "Short and stout"
}
}

With a classic App Service you could override a setting in the appsettings.json by updating "Application settings" within the Azure portal. You'd do this in the style of creating an Application setting called Parent:ChildOne or Parent:ChildTwo. To be clear: using colons to target a specific piece of config.

You can read about this approach here. Now there's something I want you to notice; consider the colons below:

If you try and follow the same steps when you're using Web App for Containers / i.e. a Docker image deployed to an Azure App Service on Linux you cannot use colons:

When you hover over the error you see this message: This field can only contain letters, numbers (0-9), periods ("."), and underscores ("_"). Using . does not work alas.

What do I do?#

So it turns out you just can't configure App Services on Linux.

Jokes!

No, of course you can and here I can help. After more experimentation than I'd like to admit I happened upon the answer. Here it is:

Where you use : on a classic App Service, you should use a __ (double underscore) on an App Service with containers. So Parent__ChildOne instead of Parent:ChildOne. It's as simple as that.

Why is it like this?#

Honestly? No idea. I can't find any information on the matter. Let me know if you find out.