Re #6531 Fix 4095-utf8-pure-nix test when in Docker container

tests/integration/tests/4095-utf8-pure-nix/Main.hs

@@ -1,5 +1,9 @@
 import StackTest
 
+import Control.Monad ( unless )
+import Data.Maybe ( isJust )
+import System.Environment ( lookupEnv )
+
 -- This test requires that Nix is installed and that the NIX_PATH has been set
 -- so as to allow the path <nixpkgs> to be used.
 main :: IO ()
@@ -11,5 +15,11 @@ main
       logInfo "Disabled on macOS as it takes too long to run, since it tries \
               \to build GHC."
   | otherwise = do
-       stack ["build", "--nix-pure"]
-       stack ["exec", "--nix-pure", "ShowUnicode"]
+      isInContainer <- getInContainer
+      unless isInContainer $ do
+         stack ["build", "--nix-pure"]
+         stack ["exec", "--nix-pure", "ShowUnicode"]
+
+-- | 'True' if we are currently running inside a Docker container.
+getInContainer :: IO Bool
+getInContainer = isJust <$> lookupEnv "STACK_IN_CONTAINER"