docker exec -it CONTAINERID sh
/app # telnet
sh: telnet: not found
/app # apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.16/main/aarch64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.16/community/aarch64/APKINDEX.tar.gz
v3.16.4-66-g2c3dfa6d54d [https://dl-cdn.alpinelinux.org/alpine/v3.16/main]
v3.16.4-66-g2c3dfa6d54d [https://dl-cdn.alpinelinux.org/alpine/v3.16/community]
OK: 16894 distinct packages available
/app # apk add busybox-extras
(1/1) Installing busybox-extras (1.35.0-r17)
Executing busybox-extras-1.35.0-r17.post-install
Executing busybox-1.35.0-r17.trigger
OK: 56 MiB in 43 packages
/app # telnet
BusyBox v1.35.0 (2022-08-01 15:14:44 UTC) multi-call binary.
Usage: telnet [-a] [-l USER] HOST [PORT]
Connect to telnet server
-a Automatic login with $USER variable
-l USER Automatic login as USER
Enjoy! 😀