Table of contents
Understanding the Challenge
This challenge gives us a link to a docker image: https://hub.docker.com/r/anssi/fcsc2024-forensics-layer-cake-1.
An environment variable containing the flag was used during the build.
Solution
Looking at the image layers at https://hub.docker.com/layers/anssi/fcsc2024-forensics-layer-cake-1/latest/images/sha256-e076eb7bc9ef18441fef7e73a08a305c5a1b631dd6789d0fc4f75c25d8c225b3?context=explore we find the ARG FIRST_FLAG which is equal to the flag
FCSC{a1240d90ebeed7c6c422969ee529cc3e1046a3cf337efe51432e49b1a27c6ad2}