Unable to run even though I appear to have all relevant features except CONFIG_USER_NS

Hi,

I'm trying to install a Flatpak app on a distro which predates Flatpak support.

Error message:

`Creating new namespace failed, likely because the kernel does not support user namespaces. bwrap must be installed setuid on such systems.`

The kernel has got the following:
CONFIG_NAMESPACES=y
CONFIG_UTS_NS=y
CONFIG_IPC_NS=y
CONFIG_PID_NS=y
CONFIG_NET_NS=y
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y
CONFIG_OVERLAYFS_FS=m
(There may be a spelling error in CONFIG_OVERLAYFS_FS in issue #87.)

`-rwsr-xr-x 1 root root 164080 Jan 6 04:55 /usr/local/bin/bwrap*`

Version is `bubblewrap 0.1.5`.

Any suggestions?