hotfix: do not call docker with --tty, it crashes under systemd

Any `docker run --tty` (which `-it` implies) crashes with:

    the input device is not a TTY

... when running as a systemd service. It seems like keeping
`-i` (which we expand to `--interactive` for clarity) keeps everything
normal (ie. we still see the container output).

Closes: #15

dangerzone-webdav-processor

@@ -246,7 +246,7 @@ class DockerRunner(object):
             cmd = [
                 "docker",
                 "run",
-                "-it",
+                "--interactive",
                 f"--cidfile={tmpdir}/cidfile",  # to get the container ID
             ]
             cmd += docker_args