How to read files and stdout from a running Docker container
You can view the filesystem of the container at
/var/lib/docker/devicemapper/mnt/$CONTAINER_ID/rootfs/
and you can just
tail -f mylogfile.log
A bit late but this is what I'm doing with journald. It's pretty powerful.
You need to be running your docker containers on an OS with systemd-journald.
docker run -d --log-driver=journald myapp
This pipes the whole lot into host's journald which takes care of stuff like log pruning, storage format etc and gives you some cool options for viewing them:
journalctl CONTAINER_NAME=myapp -f
which will feed it to your console as it is logged,
journalctl CONTAINER_NAME=myapp > output.log
which gives you the whole lot in a file to take away, or
journalctl CONTAINER_NAME=myapp --since=17:45
Plus you can still see the logs via docker logs .... if that's your preference.
No more > my.log or -v "/apps/myapp/logs:/logs" etc
To view the stdout, you can start the docker container with -i. This of course does not enable you to leave the started process and explore the container.
docker start -i containerid
Alternatively you can view the filesystem of the container at
/var/lib/docker/containers/containerid/root/
However neither of these are ideal. If you want to view logs or any persistent storage, the correct way to do so would be attaching a volume with the -v switch when you use docker run. This would mean you can inspect log files either on the host or attach them to another container and inspect them there.
The stdout of the process started by the docker container is available through the docker logs $containerid command (use -f to keep it going forever). Another option would be to stream the logs directly through the docker remote API.
For accessing log files (only if you must, consider logging to stdout or other standard solution like syslogd) your only real-time option is to configure a volume (like Marcus Hughes suggests) so the logs are stored outside the container and available for processing from the host or another container.
If you do not need real-time access to the logs, you can export the files (in tar format) with docker export