Why is it better not to store data inside a container?

Because a container is temporary and easily replaceable. If you store data inside it, that data disappears when the container is re-created or updated.

#3 reasons, junior-style

1. Containers are not permanent A container can be stopped, removed, or re-created during a deployment. Its file system is temporary, so it is easy to lose data.
2. It cannot be scaled If an application runs in several containers, the data inside each one will be different. A shared storage point is needed (a volume, a database, the cloud).
3. There is no proper backup With a container it is inconvenient to make backups, move data, and restore it.

#The right way

Data is moved out to:

• a volume (docker volume)
• a bind mount (a host folder)
• a database outside the container
• cloud storage

Summary: a container is easy to replace, while data must live longer than the container, which is why it is stored outside it, not inside.