fix environment variable name validation

Author: baronfel

Microsoft.NET.Build.Containers/ContainerHelpers.cs

@@ -24,7 +24,10 @@ public static class ContainerHelpers
 
     public const string HostObjectPass = "SDK_CONTAINER_REGISTRY_PWORD";
 
-    private static Regex envVarRegex = new Regex(@"^[a-zA-Z_]+$");
+    /// <summary>
+    /// Matches an environment variable name - must start with a letter or underscore, and can only contain letters, numbers, and underscores.
+    /// </summary>
+    private static Regex envVarRegex = new Regex(@"^[a-zA-Z_]{1,}[a-zA-Z0-9_]*$");
 
     /// <summary>
     /// DefaultRegistry is the canonical representation of something that lives in the local docker daemon. It's used as the inferred registry for repositories

Test.Microsoft.NET.Build.Containers/ContainerHelpersTests.cs

@@ -97,4 +97,18 @@ public void CanParsePort(string input, bool shouldParse, int? expectedPortNumber
             Assert.AreEqual(expectedError, errors);
         }
     }
+
+    [TestMethod]
+    [DataRow("FOO", true)]
+    [DataRow("foo_bar", true)]
+    [DataRow("foo-bar", false)]
+    [DataRow("foo.bar", false)]
+    [DataRow("foo bar", false)]
+    [DataRow("1_NAME", false)]
+    [DataRow("ASPNETCORE_URLS", true)]
+    [DataRow("ASPNETCORE_URLS2", true)]
+    public void CanRecognizeEnvironmentVariableNames(string envVarName, bool isValid) {
+        var success = ContainerHelpers.IsValidEnvironmentVariable(envVarName);
+        Assert.AreEqual(isValid, success, $"Expected {envVarName} to be {(isValid ? "valid" : "invalid")}");
+    }
 }