[msbuild] Compute _AppContainerDir from AppBundleDir if AppBundleDir is set by the developer. Fixes #15014. (#15869)

Fixes https://github.com/xamarin/xamarin-macios/issues/15014.

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -2684,6 +2684,9 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			<_AppContainerDir Condition="'$(IsAppDistribution)' == 'true'">$(ArchivePath)\Products\Applications\</_AppContainerDir>
 			<AppBundleDir>$(_AppContainerDir)$(_AppBundleName)$(AppBundleExtension)</AppBundleDir>
 		</PropertyGroup>
+		<PropertyGroup Condition="'$(_AppContainerDir)' == ''">
+			 <_AppContainerDir>$([System.IO.Path]::GetDirectoryName($(AppBundleDir)))/</_AppContainerDir>
+		</PropertyGroup>
 		<PropertyGroup>
 			<!-- Ensure _AppBundlePath is a relative path (relative to the project directory) and contains a trailing slash -->
 			<_AppBundlePath>$(AppBundleDir)</_AppBundlePath>

tests/dotnet/UnitTests/ProjectTest.cs

@@ -982,5 +982,19 @@ namespace Xamarin.Tests {
 				Assert.That (errors [0].Message, Does.Contain ("Error loading Entitlements.plist template 'Entitlements.plist'"), "Message");
 			}
 		}
+
+		[TestCase (ApplePlatform.MacOSX, "osx-arm64")]
+		public void CustomAppBundleDir (ApplePlatform platform, string runtimeIdentifiers)
+		{
+			var project = "MySimpleApp";
+			Configuration.IgnoreIfIgnoredPlatform (platform);
+
+			var project_path = GetProjectPath (project, runtimeIdentifiers: runtimeIdentifiers, platform: platform, out var appPath);
+			Clean (project_path);
+			var properties = GetDefaultProperties (runtimeIdentifiers);
+			var customAppBundleDir = Path.Combine (Cache.CreateTemporaryDirectory (), project + ".app");
+			properties ["AppBundleDir"] = customAppBundleDir;
+			var result = DotNet.AssertBuild (project_path, properties);
+		}
 	}
 }