fix #7916 by using DOTNET_RUNNING_IN_CONTAINERS env var to detect container (#9903)

src/DataProtection/DataProtection/src/Internal/ContainerUtils.cs

@@ -9,15 +9,15 @@ using System.Runtime.InteropServices;
 
 namespace Microsoft.AspNetCore.DataProtection.Internal
 {
-    internal static class DockerUtils
+    internal static class ContainerUtils
     {
-        private static Lazy<bool> _isDocker = new Lazy<bool>(IsProcessRunningInDocker);
+        private static Lazy<bool> _isContainer = new Lazy<bool>(IsProcessRunningInContainer);
 
-        public static bool IsDocker => _isDocker.Value;
+        public static bool IsContainer => _isContainer.Value;
 
         public static bool IsVolumeMountedFolder(DirectoryInfo directory)
         {
-            if (!IsDocker)
+            if (!IsContainer)
             {
                 return false;
             }
@@ -77,14 +77,21 @@ namespace Microsoft.AspNetCore.DataProtection.Internal
             return false;
         }
 
-        private static bool IsProcessRunningInDocker()
+        private static bool IsProcessRunningInContainer()
         {
+            // Official .NET Core images (Windows and Linux) set this. So trust it if it's there.
+            if(string.Equals(Environment.GetEnvironmentVariable("DOTNET_RUNNING_IN_CONTAINERS"), "true", StringComparison.OrdinalIgnoreCase))
+            {
+                return true;
+            }
+
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
             {
                 // we currently don't have a good way to detect if running in a Windows container
                 return false;
             }
 
+            // Try to detect docker using the cgroups process 1 is in.
             const string procFile = "/proc/1/cgroup";
             if (!File.Exists(procFile))
             {

src/DataProtection/DataProtection/src/Repositories/FileSystemXmlRepository.cs

@@ -31,7 +31,7 @@ namespace Microsoft.AspNetCore.DataProtection.Repositories
 
             try
             {
-                if (DockerUtils.IsDocker && !DockerUtils.IsVolumeMountedFolder(Directory))
+                if (ContainerUtils.IsContainer && !ContainerUtils.IsVolumeMountedFolder(Directory))
                 {
                     // warn users that keys may be lost when running in docker without a volume mounted folder
                     _logger.UsingEphemeralFileSystemLocationInContainer(Directory.FullName);

src/DataProtection/DataProtection/test/ContainerUtilsTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.IO;
@@ -8,7 +8,7 @@ using Xunit;
 
 namespace Microsoft.AspNetCore.DataProtection.Test
 {
-    public class DockerUtilsTests
+    public class ContainerUtilsTests
     {
         // example of content from /proc/self/mounts
         private static readonly string[] fstab = new []
@@ -37,7 +37,7 @@ namespace Microsoft.AspNetCore.DataProtection.Test
         [InlineData("../dir")]
         public void DeterminesFolderIsNotMounted(string directory)
         {
-            Assert.False(DockerUtils.IsDirectoryMounted(new DirectoryInfo(directory), fstab));
+            Assert.False(ContainerUtils.IsDirectoryMounted(new DirectoryInfo(directory), fstab));
         }
 
         [ConditionalTheory]
@@ -50,7 +50,7 @@ namespace Microsoft.AspNetCore.DataProtection.Test
         [InlineData("/app/subdir/two/")]
         public void DeterminesFolderIsMounted(string directory)
         {
-            Assert.True(DockerUtils.IsDirectoryMounted(new DirectoryInfo(directory), fstab));
+            Assert.True(ContainerUtils.IsDirectoryMounted(new DirectoryInfo(directory), fstab));
         }
     }
 }