It depends on your OS. Here are the few locations, with commands for few Operating Systems:
·         Ubuntu (old using upstart ) - /var/log/upstart/docker.log
·         Ubuntu (new using systemd ) - sudo journalctl -fu docker.service
·         Boot2Docker - /var/log/docker.log
·         Debian GNU/Linux - /var/log/daemon.log
·         CentOS - /var/log/daemon.log | grep docker
·         CoreOS - journalctl -u docker.service
·         Fedora - journalctl -u docker.service
·         Red Hat Enterprise Linux Server - /var/log/messages | grep docker
·         OpenSuSE - journalctl -u docker.service
·         OSX - ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/log/docker.log
Windows - Get-EventLog -LogName Application -Source Docker -After (Get-Date).AddMinutes(-5) | Sort-Object Time