Discourse upgrade fails due to insufficient disk space on 25G droplet

installation
2 
root@DO-Discourse:/var/discourse# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            448M     0  448M   0% /dev
tmpfs            99M  1.1M   98M   2% /run
/dev/vda1        25G   18G  6.2G  75% /
tmpfs           491M     0  491M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           491M     0  491M   0% /sys/fs/cgroup
/dev/vda15      105M  9.2M   96M   9% /boot/efi
overlay          25G   18G  6.2G  75% /var/lib/docker/overlay2/d319d95263d87c2a75a4bc9a9f03a25ea7f6eb1f7bac687e7ae7d45522939dc0/merged
/dev/loop10      56M   56M     0 100% /snap/core18/2284
/dev/loop7       44M   44M     0 100% /snap/snapd/14978
/dev/loop5       62M   62M     0 100% /snap/core20/1361
/dev/loop2       68M   68M     0 100% /snap/lxd/22526
/dev/loop1       62M   62M     0 100% /snap/core20/1376
/dev/loop3       44M   44M     0 100% /snap/snapd/15177
/dev/loop4       56M   56M     0 100% /snap/core18/2344
/dev/loop6       68M   68M     0 100% /snap/lxd/22753
tmpfs            99M     0   99M   0% /run/user/0
root@DO-Discourse:/var/discourse#