As a simple lay programmer, I sometimes have trouble figuring out where log files are stored on unix systems. Sometimes logs are within application directories. Other times they are in /var/log. With Docker containers, this uncertainty is eliminated. How? By the 'docker diff' command. I will show why. When connecting to a Docker-based system, you can see the running containers:

$ docker ps
CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS              PORTS                                                                                                                                                                                                                                                                                                                                    NAMES
90a9f7122c02        medined/accumulo:latest   /run.sh walt        9 hours ago         Up 9 hours          0.0.0.0:49153->50070/tcp, 0.0.0.0:49154->50090/tcp, 0.0.0.0:49155->50095/tcp, 0.0.0.0:49156->8025/tcp, 0.0.0.0:49157->8030/tcp, 0.0.0.0:49158->8088/tcp, 0.0.0.0:49159->10020/tcp, 0.0.0.0:49160->19888/tcp, 0.0.0.0:49161->2181/tcp, 0.0.0.0:49162->22/tcp, 0.0.0.0:49163->8020/tcp, 0.0.0.0:49164->8050/tcp, 0.0.0.0:49165->8141/tcp   walt

Then you can list changed files within the container using the image id or name.

$ docker diff walt
...
D /data1/hdfs/dn/current/BP-1274135865-172.17.0.10-1404767453280/current/finalized/blk_1073741825_1001.meta
...
A /var/log/supervisor/accumulo-gc-stderr---supervisor-5H7Rr7.log
A /var/log/supervisor/accumulo-gc-stdout---supervisor-LK8wDU.log
...
A /var/log/supervisor/namenode-stdout---supervisor-mciN4u.log
A /var/log/supervisor/secondarynamenode-stderr---supervisor-EaluLZ.log
A /var/log/supervisor/secondarynamenode-stdout---supervisor-Ap4Fri.log
C /var/log/supervisor/supervisord.log
A /var/log/supervisor/zookeeper-stderr---supervisor-CCwUGw.log
A /var/log/supervisor/zookeeper-stdout---supervisor-lDiuIF.log
C /var/run
C /var/run/sshd.pid
C /var/run/supervisord.pid

Armed with this list you can confidently either look in /var/lib/docker or use the nsenter command to join the namespace of the container to read interesting files.