rel=$(lsb_release -sc); 
mirror="http://archive.ubuntu.com/ubuntu"
lfile="/etc/apt/sources.list.d/$rel-proposed.list"
packages="ubuntu-cloudimage-keyring"

sjson_url="http://cloud-images.ubuntu.com/releases/streams/v1/index.sjson"
keyring="/usr/share/keyrings/ubuntu-cloudimage-keyring.gpg"

echo "deb $mirror $rel-proposed main" | sudo tee $lfile >/dev/null

vrun() { echo "$" "$@" 1>&2; "$@"; echo; }

vrun cat $lfile
vrun sudo apt-get update --quiet >/dev/null 2>&1
vrun apt-cache policy $packages

vrun sudo DEBIAN_FRONTEND=noninteractive apt-get install \
    --assume-yes --quiet $packages

vrun wget -q "$sjson_url" -O file.sjson

echo "# demonstrate that not signed by a key elsewhere"
vrun gpg --verify file.sjson

echo "# verify it is signed by something in $keyring"
vrun gpg --keyring $keyring --verify file.sjson