Fix github actions run and remove the docker container after test

Not sure why the docker container was started with "-it", but the CI run on Github Actions failed like so:

TestEnvironmentVariables::AsIfContinuousIntegration:
the input device is not a TTY
  test_RAKE_EXTENSION_TASK_NO_NATIVE:			F
===============================================================================
Failure: test_RAKE_EXTENSION_TASK_NO_NATIVE(TestEnvironmentVariables::AsIfContinuousIntegration)
/home/runner/work/rake-compiler-dock/rake-compiler-dock/test/test_environment_variables.rb:35:in `test_RAKE_EXTENSION_TASK_NO_NATIVE'
     32:     end
     33:
     34:     def test_RAKE_EXTENSION_TASK_NO_NATIVE
  => 35:       assert_equal "true", rcd_env['RAKE_EXTENSION_TASK_NO_NATIVE']
     36:     end
     37:
     38:     def test_symlink_rake_compiler
<"true"> expected but was
<nil>

Author: larskanis

test/test_environment_variables.rb

@@ -75,7 +75,7 @@ class AsIfContinuousIntegration < Test::Unit::TestCase
     include Common
 
     def invocation(command)
-      "docker run -it #{TEST_IMAGE_NAME} bash -c '#{command}'"
+      "docker run --rm #{TEST_IMAGE_NAME} bash -c '#{command}'"
     end
   end
 end