add an error that prevents usage on unsupported SDKs

Author: baronfel

Microsoft.NET.Build.Containers/build/Microsoft.NET.Build.Containers.targets

@@ -1,9 +1,16 @@
 <Project>
-    <Target Name="_ContainerVerifySDKVersion">
+    <Target Name="_ContainerVerifySDKVersion"
+        Condition="'$(WebPublishMethod)' == 'Container' or '$(PublishProfile)' == 'DefaultContainer'"
+        BeforeTargets="AfterPublish">
+        <!-- If the user has opted into container publishing via their own profile (WebPublishMethod = Container) or
+            via the default Profile (PublishProfile = DefaultContainer), make sure they're on a supported SDK version.
+            We do the explicit profile name check here because for preview6 for example the profile didn't exist, so we
+            can't rely only on the WebPublishMethod. -->
         <PropertyGroup>
-            <_IsAllowedVersion Condition="$(NETCoreSdkVersion.StartsWith('7.0.100-preview.7')) or $(NETCoreSdkVersion.StartsWith('7.0.100-rc')) or $(NETCoreSdkVersion.StartsWith('7.0.100'))">true</_IsAllowedVersion>
+            <!-- Allow preview 7, any RC, or any stable version of 7 -->
+            <_IsAllowedVersion Condition="$(NETCoreSdkVersion.StartsWith('7.0.100-preview.7')) or $(NETCoreSdkVersion.StartsWith('7.0.100-rc')) or ($(NETCoreSdkVersion.StartsWith('7.0.10')) and $(NETCoreSdkVersion.Contains('-')) == false)">true</_IsAllowedVersion>
         </PropertyGroup>
-        <Error Code="CONTAINER002" Text="The current .NET SDK \($(NETCoreSdkVersion)\) doesn't support containerization. Please use version 7.0.100-preview.7 or higher." />
+        <Error Condition="'$(_IsAllowedVersion)' != 'true'" Code="CONTAINER002" Text="The current .NET SDK ($(NETCoreSdkVersion)) doesn't support containerization. Please use version 7.0.100-preview.7 or higher." />
     </Target>
 
     <Target Name="ComputeContainerConfig">