Add s390x Ubuntu image

See ghc#20343.

images/debian.dhall

@@ -273,6 +273,17 @@ let images: List Image.Type =
     , extraPackages = ubuntuBuildDepends
     }
 
+, DebianImage.toDocker DebianImage::
+    { name = "s390x-linux-ubuntu20_04"
+    , fromImage = "s390x/ubuntu:20.04"
+    , runnerTags = [ "s390x-linux" ]
+    , bootstrapLlvm = None Llvm.Source
+    , bootstrapGhc = { version = "8.10.4", triple = "s390x-deb9-linux" }
+    , llvm = Some (Llvm.Source.FromBindist { version = "11.0.1" , triple = "s390x-linux-gnu-ubuntu-16.04" })
+    , cabalSource = Cabal.fromUpstreamBindist { version = "3.2.0.0", triple = "s390x-unknown-linux" }
+    , extraPackages = ubuntuBuildDepends
+    }
+
 , Image::
   { name = "linters"
   , runnerTags = [ "x86_64-linux" ]