This is taken from the official docker website Dockerfile best practices ..
VOLUME
The VOLUME instruction should be used to expose any database storage area, configuration storage, or files/folders created by your docker container. You are strongly encouraged to use VOLUME for any mutable and/or user-serviceable parts of your image.
What is meant by using volume for any mutable and user serviceable parts of the image? Are there times when I should/shouldnt and use a volume for databases? If so why? Is this where you mount the actual data contents of the database separate from the docker container..
Not a complete answer, but I found an example which might help. From the book "Build your own PAAS with Docker" by Oskar Hane, where he creates a container used only to host files for other containers, such as a MySQL container:
There is a VOLUME instruction for the Dockerfile, where you can define which directories to expose to other containers when this data volume container is added using --volumes-from attribute. In our data volume containers, we first need to add a directory for MySQL data. Let's take a look inside the MySQL image we will be using to see which directory is used for the data storage, and expose that directory to our data volume container so that we can own it:
RUN mkdir –p /var/lib/mysql
VOLUME ["/var/lib/mysql"]