Once vscode is connected to the remote container, execute the following commands inside the vscode terminal.

sudo nix-env -i cabal-install
sudo nix-env -iA nixpkgs.binutils-unwrapped
sudo nix-env -i gnome-keyring
cabal update
cabal new-install --lib sort text split Unique extra containers

In VSCode install the following extension:

vscode:extension/haskell.haskell