look for .dockerenv, not Dockerfile

lib/mongo/server/app_metadata/environment.rb

@@ -42,6 +42,10 @@ class TypeMismatch < Mongo::Error; end
         # Error class for reporting that the value for a field is too long.
         class ValueTooLong < Mongo::Error; end
 
+        # The name and location of the .dockerenv file that will signal the
+        # presence of Docker.
+        DOCKERENV_PATH = '/.dockerenv'.freeze
+
         # This value is not explicitly specified in the spec, only implied to be
         # less than 512.
         MAXIMUM_VALUE_LENGTH = 500
@@ -212,7 +216,7 @@ def detect_environment
         end
 
         # Looks for the presence of a container. Currently can detect
-        # Docker (by the existence of a Dockerfile in the working
+        # Docker (by the existence of a .dockerenv file in the root
         # directory) and Kubernetes (by the existence of the KUBERNETES_SERVICE_HOST
         # environment variable).
         def detect_container
@@ -228,7 +232,13 @@ def detect_container
 
         # Checks for the existence of a Dockerfile in the working directory.
         def docker_present?
-          File.exist?('Dockerfile')
+          File.exist?(dockerenv_path)
+        end
+
+        # Implementing this as a method so that it can be mocked in tests, to
+        # test the presence or absence of Docker.
+        def dockerenv_path
+          DOCKERENV_PATH
         end
 
         # Checks for the presence of a non-empty KUBERNETES_SERVICE_HOST

spec/mongo/server/app_metadata/environment_spec.rb

@@ -3,24 +3,34 @@
 require 'spec_helper'
 require 'fileutils'
 
+MOCKED_DOCKERENV_PATH = File.expand_path(File.join(Dir.pwd, '.dockerenv-mocked'))
+
 module ContainerChecking
+  def mock_dockerenv_path
+    before do
+      allow_any_instance_of(Mongo::Server::AppMetadata::Environment)
+        .to receive(:dockerenv_path)
+        .and_return(MOCKED_DOCKERENV_PATH)
+    end
+  end
+
   def with_docker
+    mock_dockerenv_path
+
     around do |example|
-      keep = File.exist?('Dockerfile')
-      File.write('Dockerfile', 'placeholder') unless keep
+      File.write(MOCKED_DOCKERENV_PATH, 'placeholder')
       example.run
     ensure
-      File.delete('Dockerfile') unless keep
+      File.delete(MOCKED_DOCKERENV_PATH)
     end
   end
 
   def without_docker
+    mock_dockerenv_path
+
     around do |example|
-      save = 'Dockerfile' if File.exist?('Dockerfile')
-      FileUtils.mv('Dockerfile', '_Dockerfile') if save
+      FileUtils.rm_f(MOCKED_DOCKERENV_PATH)
       example.run
-    ensure
-      FileUtils.mv('_Dockerfile', 'Dockerfile') if save
     end
   end