Why can't "docker commit" write data into new image?

Docker Engine General
1

I am using genezys/gitlab image to build a gitlab server:

docker run --name gitlab_data genezys/gitlab:7.5.2 /bin/true
docker run --detach --name gitlab --publish 8080:80 --publish 2222:22 --volumes-from gitlab_data genezys/gitlab:7.5.2

Now, I want to backup the gitlab_data container, so the code repository can be stored.

I use docker commit command to generate a new image. Then when creating the container from the new image, I find all the code repository is lost, and it seems a new empty image. I also post this issue on SO, but can’t find the root cause.

Could anyone give some clues? Thanks very much in advance!

P.S. My docker version is 1.5.

2

Data stored on “volumes” is not actually in the container: it is in a non-obvious directory on the host. Using a data container to share volumes is just a convenient, host-independent-ish way of mapping host volumes into containers. Committing a data container does not save the data, but it doesn’t lose it either – it should still be on the host and should be accessible via the original container.

To back up a data container, it would be better to copy it out onto a known volume on the host. The docs detail how to do that here: https://docs.docker.com/userguide/dockervolumes/#backup-restore-or-migrate-data-volumes

3

@rufus: Thanks very much for your comments!

But from this post, I learn the new image actually contains the new folder. Could you explain it if you have time? Thanks very much in advance!