support other channels for ComputeDotnetBaseImageTag

Author: vlada-shubina

src/Containers/Microsoft.NET.Build.Containers/Tasks/ComputeDotnetBaseImageTag.cs

@@ -97,6 +97,14 @@ public override bool Execute()
                     return null;
                 }
             }
+            else if (channel == "alpha")
+            {
+                return $"{major}.{minor}-preview.1";
+            }
+            else if (channel == "dev" || channel == "ci")
+            {
+                return $"{major}.{minor}-preview";
+            }
             else
             {
                 Log.LogError(Resources.Strings.InvalidSdkPrereleaseVersion, channel);

src/Tests/Microsoft.NET.Build.Containers.IntegrationTests/TargetsTests.cs

@@ -140,6 +140,10 @@ public void ShouldNotIncludeSourceControlLabelsUnlessUserOptsIn(bool includeSour
     [InlineData("8.0.200-preview3", "v8.0", "8.0")]
     [InlineData("6.0.100", "v6.0", "6.0")]
     [InlineData("6.0.100-preview.1", "v6.0", "6.0")]
+    [InlineData("8.0.100-dev", "v8.0", "8.0-preview")]
+    [InlineData("8.0.100-ci", "v8.0", "8.0-preview")]
+    [InlineData("8.0.100-alpha.12345", "v8.0", "8.0-preview.1")]
+    [InlineData("9.0.100-alpha.12345", "v9.0", "9.0-preview.1")]
     [Theory]
     public void CanComputeTagsForSupportedSDKVersions(string sdkVersion, string tfm, string expectedTag)
     {