One of the fist commands you want to run in your Dockerfile is certainly 'apt-get update'. I'm writing this with the hope to save the reader some precious time: if 'apt-get update' fails, you may want to ensure that docker's DNS configuration is complete (and servers are accessible).
In my case this meant simply adding the Google DNS servers in /etc/default/docker:
DOCKER_OPTS="--dns 18.104.22.168 --dns 22.214.171.124"
and restart docker with
sudo service docker restart
This StackOverflow question is related.