Mention the use of DOTNET_ReadOnlyDataProtectionKeyDirectory created by the ASP.NET Data Protection feature in ACA

### Description

Azure Container Apps provides a feature that will create a key volume mount inside the container for the scaling set to use at the location specified by the injected environment variable `DOTNET_ReadOnlyDataProtectionKeyDirectory`.

To enable it, go to the Container App in Azure Portal and select the Development Stack section on the left. Then set it to .NET and enable the "ASP .NET Core Data Protection" feature. It can be enabled through ARM bicep:
```yaml
resource containerApp 'Microsoft.App/containerApps@2024-10-02-preview'= {
properties: {
configuration: {
runtime: {
dotnet: {
autoConfigureDataProtection: true
}
}
```

The volume mount is supposedly read-only. So I'm guessing key rotation is handled by Container Apps service.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/host-and-deploy/scaling-aspnet-apps/scaling-aspnet-apps?view=aspnetcore-9.0&tabs=login-azure-cli

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/host-and-deploy/scaling-aspnet-apps/scaling-aspnet-apps.md

### Document ID

8f9736cd-5888-8c01-d2f9-e13353d0ee1c

### Platform Id

91a6ff36-d97b-db01-28a6-bfcce8a5bb0c

### Article author

@alexwolfmsft

### Metadata

* ID: 8f9736cd-5888-8c01-d2f9-e13353d0ee1c
* PlatformId: 91a6ff36-d97b-db01-28a6-bfcce8a5bb0c
* Service: **aspnet-core**
* Sub-service: **hosting-deployment**

[Related Issues](https://github.com/dotnet/AspNetCore.Docs/issues?q=is%3Aissue+is%3Aopen+8f9736cd-5888-8c01-d2f9-e13353d0ee1c)
#32530 has PR addressing this but it's much larger in scope and has been under review for over six months.