Fix: Support managed identity authentication for ACR without requiring username/password

[Copilot]

Summary

Fixes the issue where the module incorrectly required username and password credentials when using managed identity authentication to pull images from Azure Container Registry (ACR). This prevents users from deploying ACI with the recommended secure authentication method.

Problem

When deploying Azure Container Instances with a user-assigned managed identity to authenticate with ACR, the module was throwing validation errors:

Error: expected "image_registry_credential.0.username" to not be an empty string
Error: expected "image_registry_credential.0.password" to not be an empty string

This occurred even though username and password should not be required when using managed identity authentication, as shown in this working configuration with the native resource:

resource "azurerm_container_group" "example" {
  image_registry_credential {
    user_assigned_identity_id = "/subscriptions/.../userAssignedIdentities/my-identity"
    server = "myacr.azurecr.io"
    # No username or password needed!
  }
}

Root Cause

In the image_registry_credential dynamic block in main.tf, optional fields (username, password, user_assigned_identity_id) were being directly assigned from the variable values. When these optional fields were not provided by users, they would have null values, which in some cases could be interpreted as empty strings by the Azure provider, triggering validation errors.

Solution

Modified the image_registry_credential block to use the try() function for proper null handling:

dynamic "image_registry_credential" {
  for_each = var.image_registry_credential

  content {
    server                    = image_registry_credential.value.server
    password                  = try(image_registry_credential.value.password, null)
    user_assigned_identity_id = try(image_registry_credential.value.user_assigned_identity_id, null)
    username                  = try(image_registry_credential.value.username, null)
  }
}

This ensures that when optional authentication fields are not provided, they are properly handled as null and omitted from the Azure API call, allowing managed identity authentication to work correctly.

Changes

• main.tf: Updated image_registry_credential block to use try() for optional fields (3 lines changed)
• examples/acr-managed-identity/: Added new comprehensive example demonstrating ACR access with managed identity authentication
• Documentation: Auto-generated README updates

Example Usage

Users can now deploy ACI with managed identity authentication without providing credentials:

module "aci" {
  source  = "Azure/avm-res-containerinstance-containergroup/azurerm"
  
  # ... other required fields ...
  
  managed_identities = {
    user_assigned_resource_ids = [azurerm_user_assigned_identity.aci.id]
  }
  
  # ACR authentication using managed identity - no username/password!
  image_registry_credential = {
    acr = {
      server                    = "myacr.azurecr.io"
      user_assigned_identity_id = azurerm_user_assigned_identity.aci.id
    }
  }
}

Impact

• ✅ Enables managed identity authentication for ACR (security best practice)
• ✅ Maintains backward compatibility with username/password authentication
• ✅ Aligns module behavior with native azurerm_container_group resource capabilities
• ✅ Minimal change - only 3 lines modified in core module

Related Issues

Fixes #[issue_number] and addresses similar concerns raised in #30

Testing

• ✅ Terraform syntax validation passed
• ✅ Terraform formatting verified
• ✅ New example created and validated
• ✅ Documentation generated successfully

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[AVM Module Issue]: ACI Deployment with System-Assigned Identity Still Requests Credentials</issue_title>
<issue_description>### Check for previous/existing GitHub issues

• I have checked for previous/existing GitHub issues

Issue Type?

Bug

(Optional) Module Version

No response

(Optional) Correlation Id

No response

Description

While creating the ACI instance with system-assigned identity set to true, it still prompts for image_registry_credentials, specifically requiring a username and password—which shouldn't be necessary.

However, using the Terraform code below, I can successfully deploy and create an ACI instance that pulls the image from ACR without manually providing credentials.

resource "azurerm_container_group" "linux-container-group" {
  image_registry_credential {
    user_assigned_identity_id = "/subscriptions/${var.subscription_id}/resourceGroups/${var.resource_group_name}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/${var.aci_mi_name}"
    server = "${var.acr_name}.azurecr.io"
  }
}

whereas with ACI avm I get below error.

I have noticed similar issue was raised here.#30
Any updates on this.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #67

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