Add Docker testing for older and newer Ubuntu versions

test/buildtest

@@ -22,7 +22,8 @@ fi
 
 cabal install --dependencies-only --enable-tests "${flags[@]}" ||
   cabal install --dependencies-only "${flags[@]}" ||
-    die "can't install dependencies"
+    cabal install --dependencies-only --max-backjumps -1 "${flags[@]}" ||
+      die "can't install dependencies"
 cabal configure --enable-tests "${flags[@]}" ||
   die "configure failed"
 cabal build ||

test/distrotest

@@ -67,7 +67,10 @@ fedora:latest         dnf install -y cabal-install ghc-template-haskell-devel fi
 archlinux:latest      pacman -S -y --noconfirm cabal-install ghc-static base-devel
 
 # Ubuntu LTS
+ubuntu:22.04          apt-get update && apt-get install -y cabal-install
 ubuntu:20.04          apt-get update && apt-get install -y cabal-install
+ubuntu:18.04          apt-get update && apt-get install -y cabal-install
+ubuntu:16.04          apt-get update && apt-get install -y cabal-install
 
 # Stack on Ubuntu LTS
 ubuntu:20.04          set -e; apt-get update && apt-get install -y curl && curl -sSL https://get.haskellstack.org/ | sh -s - -f && cd /mnt && exec test/stacktest