Warning - this reduces security

Find the id of the container, you can list active containers with:

pct list

Edit the container config like:

vim /etc/pve/lxc/113.conf

Add the extra lines:

lxc.apparmor.profile: unconfined
lxc.cgroup.devices.allow: a
lxc.cap.drop:

Shutdown and restart the container.

To enable aufs/overlay in the container, it actually needs to be loaded on the proxmox host, this can be done with:

modprobe aufs
modprobe overlay

And made permanent by adding aufs & overlay to /etc/modules-load.d/modules.conf:

aufs
overlay

Then restart the proxmox server.

Lastly, inside the containter, switch to overlay fs by editing/creating /etc/docker/daemon.json and adding:

{
  "storage-driver": "overlay2"
}

And restart the container and you should have full docker support in the container.