Error mounting tmpfs on /var/lib/mock/$chroot-bootstrap/root/proc in docker container

[brianjmurrell]

I am trying to build a docker image for mock building. To speed up it's usage, I want to prime it with the root_cache tarballs.

In my Dockerfile I have added a:

RUN for chroot in {opensuse-leap-15.4,rocky+epel-{8,9}}-x86_64; do \
        su - mockbuilder -c "mock -r \"$chroot\" --shell id";                         \
    done;

But when I try to build that image, mock hits an error trying to mount the bootstap /proc:

[2023-06-01T11:44:03.152Z] Step 16/18 : RUN for chroot in {opensuse-leap-15.4,rocky+epel-{8,9}}-x86_64; do         ls -l /var/cache/mock/$chroot/root_cache/;         mock -r "$chroot" --shell id;                                      ls -l /var/cache/mock/$chroot/root_cache/;     done;
[2023-06-01T11:44:03.152Z]  ---> Running in 6c0496186695
[2023-06-01T11:44:03.409Z] ls: cannot access '/var/cache/mock/opensuse-leap-15.4-x86_64/root_cache/': No such file or directory
[2023-06-01T11:44:03.976Z] INFO: mock.py version 3.5 starting (python version = 3.11.3, NVR = mock-3.5-1.fc37)...
[2023-06-01T11:44:03.976Z] Start(bootstrap): init plugins
[2023-06-01T11:44:04.231Z] INFO: selinux disabled
[2023-06-01T11:44:04.231Z] Finish(bootstrap): init plugins
[2023-06-01T11:44:04.231Z] Start: init plugins
[2023-06-01T11:44:04.231Z] INFO: selinux disabled
[2023-06-01T11:44:04.231Z] Finish: init plugins
[2023-06-01T11:44:04.232Z] INFO: Signal handler active
[2023-06-01T11:44:04.232Z] Start: run
[2023-06-01T11:44:04.232Z] ERROR: Namespace unshare failed.
[2023-06-01T11:44:04.232Z] ERROR: It seems we are running inside of Docker. Let skip unsharing.
[2023-06-01T11:44:04.232Z] ERROR: You should *not* run anything but Mock in this container. You have been warned!
[2023-06-01T11:44:09.614Z] Start(bootstrap): chroot init
[2023-06-01T11:44:09.614Z] INFO: calling preinit hooks
[2023-06-01T11:44:09.614Z] INFO: enabled root cache
[2023-06-01T11:44:09.614Z] INFO: enabled package manager cache
[2023-06-01T11:44:09.614Z] Start(bootstrap): cleaning package manager metadata
[2023-06-01T11:44:09.614Z] Finish(bootstrap): cleaning package manager metadata
[2023-06-01T11:44:09.614Z] INFO: enabled HW Info plugin
[2023-06-01T11:44:09.614Z] ERROR: Command failed: 
[2023-06-01T11:44:09.614Z]  # /bin/mount -n -t tmpfs -o rprivate tmpfs /var/lib/mock/opensuse-leap-15.4-x86_64-bootstrap/root/proc

Is what I am trying to do impossible?