Ran out of disk space

Hello,
since I’ve installed WebODM on an almost freshly installed ubuntu system I’m experiencing a noticeable decrease of available disk space. Except for an OpenDroneMap installation and some (ODM/WebODM) processed data (10GB) there’s nothing on the system (100G system partition) that’s really worth mentioning. Today, after updating WebODM and trying to process a larger set of drone imagery I ran out of memory (again).
I found out that my /var/lib/docker directory is currently 34G, even though I’ve deleted all projects, shut down WebODM, cleaned it using the webodm.sh down command and rebooting the system.

[email protected]:/var/lib/docker# du -shx *
31G     aufs
148K    containers
77M     image
68K     network
20K     plugins
4,0K    swarm
4,0K    tmp
4,0K    trust
2,6G    volumes

[email protected]:/var/lib/docker# docker volume ls
DRIVER              VOLUME NAME
local               webodm_appmedia
local               webodm_dbdata

docker images
REPOSITORY                       TAG                 IMAGE ID            CREATED             SIZE
opendronemap/webodm_webapp       latest              ecbadd5b1e8b        10 hours ago        1.68 GB
opendronemap/webodm_db           latest              b3e5e1ee95a3        10 hours ago        466 MB
opendronemap/node-opendronemap   latest              d2a5cc24c818        15 hours ago        4.29 GB
opendronemap/webodm_webapp       <none>              6aa6ce001c34        27 hours ago        1.68 GB
<none>                           <none>              7a5de060ea33        3 weeks ago         1.51 GB
<none>                           <none>              e50369a7078c        3 weeks ago         466 MB
opendronemap/node-opendronemap   <none>              b987ed91b64f        3 weeks ago         4.29 GB
opendronemap/node-opendronemap   <none>              58cf4d797a7a        3 weeks ago         4.29 GB
opendronemap/node-opendronemap   <none>              154be8a68d4a        3 months ago        4.26 GB
opendronemap/webodm_webapp       <none>              2654f58a6ca3        4 months ago        1.5 GB
opendronemap/webodm_db           <none>              7cf185d87821        5 months ago        609 MB
webodm_webapp                    latest              2be437d23e51        5 months ago        1.45 GB
webodm_db                        latest              3ac6fa7aecd1        5 months ago        609 MB
<none>                           <none>              cdc196411d8a        5 months ago        1.45 GB
<none>                           <none>              53ecc5ea9cc0        5 months ago        609 MB
<none>                           <none>              08eca7057352        5 months ago        1.45 GB
<none>                           <none>              264e984a4b69        5 months ago        609 MB
opendronemap/node-opendronemap   <none>              1007aa757c24        6 months ago        4.19 GB
opendronemap/opendronemap        latest              3be5c93c5725        6 months ago        3.78 GB
postgres                         9.5                 285433a9049d        6 months ago        268 MB
python                           3.5                 538287ecca03        6 months ago        689 MB

What’s going wrong here and how do I free up those 31G of disk space in the /var/lib/docker/aufs directory (docker images)?

Thanks in advance!

Probably some unused containers, images and/or volumes. See https://gist.github.com/bastman/5b57ddb3c11942094f8d0a97d461b430