feat: Enhance image name handling and add unit tests for absolute image references

app/Infrastructure/AirlockRunner.cs

@@ -69,7 +69,7 @@ public static int Run(
         DebugLogger.Log($"Using external network: {externalNetwork}");
     }
 
-    var appImage = $"{ImagePrefix}:{imageTag}";
+    var appImage = ContainerRunner.IsAbsoluteImageReference(imageTag) ? imageTag : $"{ImagePrefix}:{imageTag}";
     var proxyImage = $"{ImagePrefix}:proxy";
 
     Console.WriteLine("🛡️  Starting in Airlock mode...");

app/Infrastructure/ContainerRunner.cs

@@ -11,8 +11,16 @@ public static class ContainerRunner
 
   /// <summary>
   /// Gets the full image name for a given tag.
+  /// If the tag contains a '/', it is treated as an absolute image reference and used as-is.
   /// </summary>
-  public static string GetImageName(string tag) => $"{ImagePrefix}:{tag}";
+  public static string GetImageName(string tag) =>
+    IsAbsoluteImageReference(tag) ? tag : $"{ImagePrefix}:{tag}";
+
+  /// <summary>
+  /// Determines whether a value is an absolute image reference (e.g., "myregistry.io/myimage:v1")
+  /// rather than a simple tag (e.g., "dotnet-rust").
+  /// </summary>
+  public static bool IsAbsoluteImageReference(string value) => value.Contains('/');
 
   /// <summary>
   /// Runs a container command with the given arguments.

app/Tools/EchoTool.cs

@@ -13,6 +13,10 @@ public class EchoTool : Infrastructure.ICliTool
 
     public string GetImageName(string tag)
     {
+        // If the tag is an absolute image reference (contains '/'), use it as-is
+        if (Infrastructure.ContainerRunner.IsAbsoluteImageReference(tag))
+            return tag;
+
         // Echo uses the same images as GitHub Copilot (copilot-* tags)
         // It doesn't need separate images - it just echoes config instead of running the tool
         const string imagePrefix = "ghcr.io/gordonbeeming/copilot_here";

app/Tools/GitHubCopilotTool.cs

@@ -16,6 +16,10 @@ public sealed class GitHubCopilotTool : ICliTool
 
   public string GetImageName(string tag)
   {
+    // If the tag is an absolute image reference (contains '/'), use it as-is
+    if (ContainerRunner.IsAbsoluteImageReference(tag))
+      return tag;
+
     // Image name format: ghcr.io/gordonbeeming/copilot_here:copilot-{variant}
     // Tag matches what users invoke: "copilot" (not "github-copilot")
     const string imagePrefix = "ghcr.io/gordonbeeming/copilot_here";

tests/CopilotHere.UnitTests/ContainerRunnerImageTests.cs

@@ -0,0 +1,93 @@
+using CopilotHere.Infrastructure;
+using TUnit.Core;
+
+namespace CopilotHere.Tests;
+
+public class ContainerRunnerImageTests
+{
+  // === IsAbsoluteImageReference ===
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithSimpleTag_ReturnsFalse()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("latest")).IsFalse();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithDotnetTag_ReturnsFalse()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("dotnet-rust")).IsFalse();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithRegistryImage_ReturnsTrue()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("myregistry.io/myimage:v1")).IsTrue();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithDockerHubImage_ReturnsTrue()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("myuser/myimage:latest")).IsTrue();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithGhcrImage_ReturnsTrue()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("ghcr.io/myorg/myimage:v2")).IsTrue();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithLocalRegistryImage_ReturnsTrue()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("localhost:5000/myimage:dev")).IsTrue();
+  }
+
+  [Test]
+  public async Task IsAbsoluteImageReference_WithNestedPath_ReturnsTrue()
+  {
+    await Assert.That(ContainerRunner.IsAbsoluteImageReference("registry.example.com/org/team/image:tag")).IsTrue();
+  }
+
+  // === GetImageName ===
+
+  [Test]
+  public async Task GetImageName_WithSimpleTag_ReturnsFullImageWithPrefix()
+  {
+    // Act
+    var imageName = ContainerRunner.GetImageName("latest");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("ghcr.io/gordonbeeming/copilot_here:latest");
+  }
+
+  [Test]
+  public async Task GetImageName_WithDotnetTag_ReturnsFullImageWithPrefix()
+  {
+    // Act
+    var imageName = ContainerRunner.GetImageName("dotnet-rust");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("ghcr.io/gordonbeeming/copilot_here:dotnet-rust");
+  }
+
+  [Test]
+  public async Task GetImageName_WithAbsoluteImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = ContainerRunner.GetImageName("myregistry.io/myimage:v1");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myregistry.io/myimage:v1");
+  }
+
+  [Test]
+  public async Task GetImageName_WithDockerHubImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = ContainerRunner.GetImageName("myuser/custom-copilot:latest");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myuser/custom-copilot:latest");
+  }
+}

tests/CopilotHere.UnitTests/EchoToolTests.cs

@@ -42,6 +42,26 @@ public async Task GetImageName_WithDotnet_ReturnsCorrectFormat()
     await Assert.That(imageName).IsEqualTo("ghcr.io/gordonbeeming/copilot_here:copilot-dotnet");
   }
 
+  [Test]
+  public async Task GetImageName_WithAbsoluteImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = _tool.GetImageName("myregistry.io/custom-image:v1");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myregistry.io/custom-image:v1");
+  }
+
+  [Test]
+  public async Task GetImageName_WithDockerHubAbsoluteImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = _tool.GetImageName("myuser/custom-image:latest");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myuser/custom-image:latest");
+  }
+
   [Test]
   public async Task GetDockerfile_ReturnsCorrectPath()
   {

tests/CopilotHere.UnitTests/GitHubCopilotToolTests.cs

@@ -52,6 +52,26 @@ public async Task GetImageName_WithEmptyTag_ReturnsDefault()
     await Assert.That(imageName).IsEqualTo("ghcr.io/gordonbeeming/copilot_here:copilot-latest");
   }
 
+  [Test]
+  public async Task GetImageName_WithAbsoluteImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = _tool.GetImageName("myregistry.io/custom-copilot:v1");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myregistry.io/custom-copilot:v1");
+  }
+
+  [Test]
+  public async Task GetImageName_WithDockerHubAbsoluteImage_ReturnsAsIs()
+  {
+    // Act
+    var imageName = _tool.GetImageName("myuser/copilot-custom:latest");
+
+    // Assert
+    await Assert.That(imageName).IsEqualTo("myuser/copilot-custom:latest");
+  }
+
   [Test]
   public async Task GetDockerfile_ReturnsCorrectPath()
   {