More informative docker logs output

Example log output with the modifications in this pr:
```
$ docker logs <container-id>
==> db: 140620 11:58:06 mysqld_safe Logging to '/var/log/mysql.err'.
==> db: 140620 11:58:06 mysqld_safe Starting mysqld daemon with databases from /opt/mysql
==> db: ==> /var/log/mysql.log <==
==> db:
==> db: ==> /var/log/mysql.err <==
==> db: InnoDB: Restoring possible half-written data pages from the doublewrite
==> db: InnoDB: buffer...
==> db: InnoDB: Last MySQL binlog file position 0 1250803, file name /var/log/mysql/mariadb-bin.000007
==> db: 140620 11:58:06  InnoDB: Waiting for the background threads to start
==> db: 140620 11:58:07 Percona XtraDB (http://www.percona.com) 5.5.37-MariaDB-35.0 started; log sequence number 26584403
==> db: 140620 11:58:07 [Note] Plugin 'FEEDBACK' is disabled.
==> db: 140620 11:58:07 [Note] Server socket created on IP: '0.0.0.0'.
==> db: 140620 11:58:07 [Note] Event Scheduler: Loaded 0 events
==> db: 140620 11:58:07 [Note] /usr/sbin/mysqld: ready for connections.
==> db: Version: '5.5.38-MariaDB-1~quantal-log'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  mariadb.org binary distribution
```