adjusted dockerfile

Dockerfile

@@ -9,7 +9,7 @@ FROM ubuntu
 # MAINTAINER name Chunteng Xiao
 RUN apt-get clean
 RUN apt-get update
-RUN apt-get install software-properties-common
+RUN apt-get install -y software-properties-common
 RUN add-apt-repository ppa:certbot/certbot
 RUN apt-get update
 RUN apt-get -f install -y nodejs nginx npm curl unzip python-certbot-nginx