Podman image: Set default_sysctls to empty for rootless containers

Avoids the error "Error: error preparing container xyz... for attach:
crun: open /proc/sys/net/ipv4/ping_group_range: Read-only file system:
OCI runtime error" when using `podman run --net bridge` inside rootful
Podman running without --security-opt unmask=ALL (or 'unmask=/proc/*')

Signed-off-by: Joakim Nohlgård <joakim@nohlgard.se>

contrib/podmanimage/stable/podman-containers.conf

@@ -2,3 +2,4 @@
 volumes = [
 	"/proc:/proc",
 ]
+default_sysctls = []