x11docker support for sysbox (userns-remap issues)

Currently I work on support for sysbox-runc in x11docker that allows to run GUI applications in container.

After installing the sysbox debian package, I found new entries for sysbox in /etc/docker/daemon.json:

{
    "graph": "/sda7docker",
    "default-runtime": "runc",
    "runtimes": {
        "kata-runtime": {
            "path": "/usr/bin/kata-runtime"
        },
        "crun": {
            "path": "/usr/local/bin/crun"
        },
        "sysbox-runc": {
            "path": "/usr/bin/sysbox-runc"
        }
    },
     "userns-remap": "sysbox"
    "bip": "172.20.0.1/16",
    "default-address-pools": [
        {
            "base": "172.25.0.0/16",
            "size": 24
        }
    ]
}

docker with sysbox-runc works well so far, but I don't have access to already existing images.
I have to remove "userns-remap": "sysbox" to get back access to my images. But than sysbox-runc doesn't work anymore.
I have tried to move the entry to sysbox-runc only, but that does not work:

{
    "graph": "/sda7docker",
    "default-runtime": "runc",
    "runtimes": {
        "kata-runtime": {
            "path": "/usr/bin/kata-runtime"
        },
        "crun": {
            "path": "/usr/local/bin/crun"
        },
        "sysbox-runc": {
            "path": "/usr/bin/sysbox-runc",
            "userns-remap": "sysbox"
        }
    },
    "bip": "172.20.0.1/16",
    "default-address-pools": [
        {
            "base": "172.25.0.0/16",
            "size": 24
        }
    ]
}

How should I configure this to have userns-remap only for sysbox-runc? Or can userns-remap be disabled at all? (It seems there is no shiftfs in debian although it is mentioned in package fuse-overlayfs.)