In order to not lose data, you would need to create a backup or snapshot of your VM. You can use the cloud provider tools for that matter. Once you have the snapshot, then, what you could do is launch a Redmine container with the same version. Then you would move the database configuration (and the database data if necessary) to the container instance.
This guide shows the steps to upgrade Redmine. These steps could be applicable to the migration case and for moving to a container. The only difference would be the location of the configuration files. In the Redmine container, they would be located in
Hope it helps
Javier J. Salmerón
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