This shows that on of the cache devices dropped offline in the past:
Jan 24 14:47:07 tower kernel: BTRFS info (device sdb1): bdev /dev/sdc1 errs: wr 1507246927, rd 137577961, flush 19733411, corrupt 0, gen 0
See here for what to do and how to better monitor a pool, then recreate the docker image.