Clear it in 3 steps
Start the environment
Investigate the target
Submit the flag

beacon.log is generated from the FLAG injected at startup. Join Base64 payloads by seq and repeat-XOR with the first eight SHA-256 bytes of beacon-v1 to recover the flag.
Inspect beacon.log and identify the target record
join Base64 payloads by seq and repeat-XOR with the first eight SHA-256 bytes of beacon-v1
Verify that the reconstructed value has FLAG{...} format