Update single.sh

[markdesilva]

Added CERTIFICATE and CERTIFICATE_KEY so users can add location for SSL certs and specified SSL certs on on docker run and

eg:

docker run --detach --restart always --publish 9392:9392 --volume /dockerfiles/sslcerts:/data/sslcerts --env HTTPS="true" --env CERTIFICATE="/data/sslcerts/fullchain.pem" --env CERTIFICATE_KEY="/data/sslcerts/privkey.pem" --name openvas immauss/openvas

This will allow users to specify a location on the docker host for the ssl certs and mount a volume to the container where the certs are located and enable HTTPS when the single.sh starts.