crashes in the container leave files in `processing`

so this is happening:

Jun 11 17:00:01 dangerzone-01 dangerzone-webdav-processor[19331]: authenticated with webdav https://nc.torproject.net/remote.php/dav/files/dangerzone-bot/
Jun 11 17:00:06 dangerzone-01 dangerzone-webdav-processor[19331]: sanitizing CVS/ 15/
Jun 11 17:00:06 dangerzone-01 dangerzone-webdav-processor[19331]: moving 15/ to CVS//dangerzone/processing/15/ before dangerzone/processing
Jun 11 17:00:08 dangerzone-01 dangerzone-webdav-processor[19331]: downloading CVS//dangerzone/processing/15/ to /tmp/tmp4te3htlj/danger/15/
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: processing 3 files in dir /tmp/tmp4te3htlj/danger/15 to safe_dir: /tmp/tmpjpzjdnxc/safe//15
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: sanitizing file /tmp/tmp4te3htlj/danger/15/nolanresume_fr.pdf into /tmp/tmpjpzjdnxc/safe//15
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: the input device is not a TTY
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: failed to run docker command: ['docker', 'run', '-it', '--cidfile=/tmp/tmpxux5tbee/cidfile', '--volum
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: Traceback (most recent call last):
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 469, in <module>
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     main()
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 336, in main
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     client.process_path(folder, path)
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 395, in process_path
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     self.sanitizer.sanitize_dir(local_path)
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 136, in sanitize_dir
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     self.sanitize_file(os.path.join(root, file), safe_dir=safe_dir)
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 145, in sanitize_file
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     args=["document-to-pixels-unpriv"],
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:   File "/usr/bin/dangerzone-webdav-processor", line 258, in run
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]:     with open(f"{tmpdir}/cidfile") as fp:
Jun 11 17:00:15 dangerzone-01 dangerzone-webdav-processor[19331]: FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpxux5tbee/cidfile'
Jun 11 17:00:15 dangerzone-01 systemd[1]: dangerzone-webdav-processor.service: Main process exited, code=exited, status=1/FAILURE

the underlying bug (the input device is not a TTY) is bad enough without those files being stuck there in the first place...

to reproduce, run under systemd with --tty, which has been patched out in 39532430, or just branch off from the parent of that.