Docker Could not resolve 'deb.debian.org'

I was trying to build a container in Docker for some experiments. Here is my Dockerfile.

FROM debianRUN mkdir -p /var/run/sshdRUN apt-get updateRUN apt-get install -y openssh-serverRUN apt-get install -y sudoRUN echo AddressFamily inet >> /etc/ssh/sshd_configARG username=RiversARG userpasswd=perfectXJ2017RUN useradd -ms /bin/bash $username && (echo $username:$userpasswd | chpasswd)RUN adduser $username sudoCMD /usr/sbin/sshd -D

I tried to build my image with the command sudo docker build -t ics-image ..

But then I got some error messages and the whole process stopped. Here is the error messages.

Sending build context to Docker daemon  2.048kBStep 1/11 : FROM debian ---> 8626492fecd3Step 2/11 : RUN mkdir -p /var/run/sshd ---> Running in 1e1f2dbbe5caRemoving intermediate container 1e1f2dbbe5ca ---> dd4bec2f81d4Step 3/11 : RUN apt-get update ---> Running in 022301215bfbGet:1 http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]Get:2 http://security.debian.org/debian-security stretch/updates/main amd64 Packages [450 kB]Err:3 http://deb.debian.org/debian stretch InRelease  Could not resolve 'deb.debian.org'Err:4 http://deb.debian.org/debian stretch-updates InRelease  Could not resolve 'deb.debian.org'Fetched 544 kB in 20s (26.9 kB/s)Reading package lists...W: Failed to fetch http://deb.debian.org/debian/dists/stretch/InRelease  Could not resolve 'deb.debian.org'W: Failed to fetch http://deb.debian.org/debian/dists/stretch-updates/InRelease  Could not resolve 'deb.debian.org'W: Some index files failed to download. They have been ignored, or old ones used instead.Removing intermediate container 022301215bfb ---> 054d32710b62Step 4/11 : RUN apt-get install -y openssh-server ---> Running in 802d2fa37b8dReading package lists...Building dependency tree...Reading state information...Package openssh-server is not available, but is referred to by another package.This may mean that the package is missing, has been obsoleted, oris only available from another sourceE: Package 'openssh-server' has no installation candidateThe command '/bin/sh -c apt-get install -y openssh-server' returned a non-zero code: 100

I did all this on Ubuntu 18.04. I cannot understand why this happened and how to solve this problem. Can anyone help?