fix: import the missing key and install nginx from upstream

tests/setup.sh

@@ -1,6 +1,10 @@
 #!/usr/bin/env bash
 set -eo pipefail
 [[ $TRACE ]] && set -x
+sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 762E3157
+echo "deb http://nginx.org/packages/ubuntu `lsb_release -cs` nginx" | sudo tee /etc/apt/sources.list.d/nginx.list
+curl -fsSL https://nginx.org/keys/nginx_signing.key | sudo apt-key add -
+
 wget https://raw.githubusercontent.com/dokku/dokku/master/bootstrap.sh
 if [[ "$DOKKU_VERSION" == "master" ]]; then
   sudo bash bootstrap.sh