Hello
As the export only works with installations of the same version, I would try the following:
1 - Build the image from the Dockerfile (exactly the version you are looking for), find it in this link Dockerfile-v6.5.1.
2 - After having built the image, create the container.
3 - Make sure that the creation of the container shows the gitlab with a base installation.
4 - create the backup in the old installation with gitlab-rake.
5 - Restore the backup in the container with gitlab-rake.
6 - Verify that the backup loaded the data correctly from the gitlab.
7 - Download the gitlab version from the docker hub.
8 - Recreate the container with the new image.
9 - Generate the backup again with gitlab-rake, in the container.
10 - Restore the backup in your new installation.
If you have any questions about the steps to take, let me know.