Disable gVisor's DirectFS feature.

DirectFS is enabled by default in gVisor to improve I/O performance,
but comes at the cost of enabling the `openat(2)` syscall (with severe
restrictions, but still). As Dangerzone is not performance-sensitive,
and that it is desirable to guarantee for the document conversion
process to not open any files (to mimic some of what SELinux provides),
might as well disable it by default.

See #226.

dangerzone/gvisor_wrapper/entrypoint.py

@@ -142,6 +142,9 @@ runsc_argv = [
     "--rootless=true",
     "--network=none",
     "--root=/home/dangerzone/.containers",
+    # Disable DirectFS for to make the seccomp filter even stricter,
+    # at some performance cost.
+    "--directfs=false",
 ]
 if os.environ.get("RUNSC_DEBUG"):
     runsc_argv += ["--debug=true", "--alsologtostderr=true"]