if I want to revert the container I can try to commit an image, and then later delete the container, and create a new container from the committed image. But if I do that the volume gets deleted and all my data is gone
As the docker user guide explains, data volumes are meant to persist data outside of a container filesystem. This also ease the sharing of data between multiple containers.
While Docker will never delete data in volumes (unless you delete the associated container with docker rm -v), volumes that are not referenced by any docker container are called dangling volumes. Those dangling volumes are difficult to get rid of and difficult to access.
This means that as soon as the last container using a volume is deleted, the data volume becomes dangling and its content difficult to access.
In order to prevent those dangling volumes, the trick is to create an additional docker container using the data volume you want to persist so that there will always be at least that docker container referencing the volume. This way you can delete the docker container running the wordpress app without losing the ease of access to that data volume content.
Such containers are called data volume containers.
There must be some simple way to back up my container plus volume data but I can’t find it anywhere.
backup docker images
To backup docker images, use the docker save command that will produce a tar archive that can be used later on to create a new docker image with the docker load command.
backup docker containers
You can backup a docker container by different means
- by committing a new docker image based on the docker container current state using the docker commit command
- by exporting the docker container file system as a tar archive using the docker export command. You can later on create a new docker image from that tar archive with the docker import command.
Be aware that those commands will only backup the docker container layered file system. This excludes the data volumes.
backup docker data volumes
To backup a data volume you can run a new container using the volume you want to backup and executing the tar command to produce an archive of the volume content as described in the docker user guide.
In your particular case, the data volume is used to store the data for a MySQL server. So if you want to export a tar archive for this volume, you will need to stop the MySQL server first. To do so you will have to stop the wordpress container.
backup the MySQL data
An other way is to remotely connect to the MySQL server to produce a database dump with the mysqldump command. However in order for this to work, your MySQL server must be configured to accept remote connections and also have a user who is allowed to connect remotely. This might not be the case with the wordpress docker image you are using.
Edit
Docker recently introduced Docker volume plugins which allow to delegate the handling of volumes to plugins implemented by vendors.
The docker run command has a new behavior for the -v option. It is now possible to pass it a volume name. Volumes created in that way are named and easy to reference later on, easing the issues with dangling volumes.
Edit 2
Docker introduced the docker volume prune command to delete all dangling volumes easily.