I’ve got docker for wsl2 tech preview running!
You need to edit /usr/sbin/enter-systemd-namespace and change line 20 to:
/bin/bash -c 'set -a; source "$HOME/.systemd-env"; set +a; exec bash -c '"$(printf "%q" "$@")"
Edit: this has broken VSCode again 
still working on it
Edit2: fixed - both docker for wsl2 tech preview and vscode remote extension are working for me with the line above (I’ve amended the original post to show the full file that I’m using)