Add the slides about when to use volumes

[refs #04fb74d73636]

Author: jfahrer

persisting_data/when_to_use_volumes1.md

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+## Volumes vs bind mounts
+* bind mounts rely on the file system structure of the host
+  * You need to manage them
+  * The container has access to the files on the Docker host
+* Volumes are managed by Docker
+  * Independent of the file system structure on the host
+  * Easy to migrate and backup
+  * Can be managed via the Docker client
+  * Can be pre-populated with data from a container

persisting_data/when_to_use_volumes2.md

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+## When to use what?
+### bind mount
+* When you want to access the data from the host and the container
+  * source code
+  * configuration files
+
+### Volume
+* When you want to persist data from the container
+* When you want to share data between containers
+  * backups
+  * configuration

persisting_data/when_to_use_volumes3.md

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+## Named vs anonymous volumes
+### Named Volume
+* You care about the data long-term
+  * A database
+  * Other services that persist data
+* You need to easily identify the volume
+  * Reuse them
+  * For backups
+  * Migrate them
+
+### Anonymous Volume
+* You don't care about the data long-term
+  * Testing
+* When you need to share data between containers
+  * using --volumes-from