Once we downloaded the container and ran it, simply runnning a Docker history command without truncation reveals the first flag.
┌──(kali㉿kali)-[/bin]
└─$ sudo docker history --no-trunc anssi/fcsc2024
IMAGE REATED CREATED BY SIZE
sha256:0faa62781dd1db0ebb6cd83836bb4ba24f8b58b0cd4 months ago CMD ["/bin/sh"] 0B
<missing> 4 months ago USER guest 0B
<missing> 4 months ago ARG FIRST_FLAG=FCSC{a1240d90ebeed7337efe51432e49b1a27c6ad2} 0B
<missing> 4 months ago /bin/sh -c #(nop) CMD ["/bin/sh"] 0B
<missing> 4 months ago /bin/sh -c #(nop) ADD file:37a76ec7431fc4085097bb6a09d81b41775473 in / 7.38MB
FCSC{a1240d90ebeed7337efe51432e49b1a27c6ad2}