如果您使用 docker run ..启动您的mysql数据库，它每次创建一个新的和新鲜的容器。您可以在 docker ps -a 的容器列表中找到旧的容器，您必须启动旧容器（ docker start< container> ）如果你想要最后一个状态。否则，您有一个从您的图像派生的新数据库。

I am new to docker world. I want to install some of the software packages into docker image. So I have taken Ubuntu:14.04 docker image and installed MySQL server there. Then did some works with it. Once I exit from docker image and commit it. But once I start docker container, databases which was created were gone. What would be the reason for this?

If you start your mysql database with docker run ..., it creates each time a new and fresh container with it. You can find your old one in the list of containers with docker ps -a and you have to start your old container (docker start <container>) if you want the last state. Otherwise you have a fresh database derived from your image.