Your problem is because of an underlying problem on the cache filesystem, the docker.img is getting remounted as read-only
Dec 23 13:58:27 Gateway kernel: BTRFS warning (device sdk1): sdk1 checksum verify failed on 1037221888 wanted 5771E135 found 6AAAD160 level 0
Dec 23 13:58:27 Gateway kernel: print_req_error: I/O error, dev loop2, sector 0
Dec 23 13:58:27 Gateway kernel: BTRFS error (device loop2): bdev /dev/loop2 errs: wr 0, rd 0, flush 1, corrupt 0, gen 0
Dec 23 13:58:27 Gateway kernel: BTRFS warning (device loop2): chunk 13631488 missing 1 devices, max tolerance is 0 for writeable mount
Dec 23 13:58:27 Gateway kernel: BTRFS: error (device loop2) in write_all_supers:3716: errno=-5 IO failure (errors while submitting device barriers.)
Dec 23 13:58:27 Gateway kernel: BTRFS info (device loop2): forced readonly
Since it's BTRFS, I'm going to bow out on the filesystem repair, as I know squat about it, but I do recommend XFS as the filesystem if you have no intent to upgrade to a cache pool