> 1. Mount current read-only directory to Docker without Internet access (and without access to local network or other processes) 2. Run `fd` 3. Print the results 4. Destroy the container

Systemd has a lot of neat sandboxing features [1] which aren't well known but can be very useful for this. You can get pretty far using systemd-run [2] in a script like this:

  #!/bin/sh
  
  uid="$(id -u)"
  gid="$(id -g)"
  cwd="$(pwd -P)"
  
  sudo systemd-run --system --pty --same-dir --wait --collect --service-type=exec \
      --uid="$uid" \
      --gid="$gid" \
      -p "TemporaryFileSystem=/:ro /tmp" \
      -p "BindReadOnlyPaths=-/bin -/sbin -/usr/bin -/usr/sbin -/lib -/lib64 -/usr/lib -/usr/lib64 -/usr/libexec" \
      -p "BindPaths=$cwd" \
      -p "PrivateNetwork=true" \
      -p "PrivateDevices=true" \
      -p "PrivateIPC=true" \
      -p "RestrictNamespaces=true" \
      -p "RestrictSUIDSGID=true" \
      -p "CapabilityBoundingSet=" \
      "$@"

Which creates a blank filesystem with no network or device access and only bind mount the specified files.

Unfortunately TemporaryFileSystem require running as a system instance of the service manager rather than per-user instance, so that will generally mean running as root (hence sudo). One approach is to create a suid binary that does the same without needing sudo.

[1] https://www.freedesktop.org/software/systemd/man/latest/syst...

[2] https://www.freedesktop.org/software/systemd/man/latest/syst...

You could also use bubblewrap [3] pretty similarly, and may not need to use sudo if unprivileged user namespaces are allowed by your kernel.

  #!/bin/sh
  
  cwd="$(pwd -P)"
  
  bwrap --new-session --die-with-parent \
      --tmpfs /tmp \
      --ro-bind-try /bin /bin \ 
      --ro-bind-try /sbin /sbin \
      --ro-bind-try /usr/bin /usr/bin \ 
      --ro-bind-try /usr/sbin /usr/sbin \
      --ro-bind-try /lib /lib \
      --ro-bind-try /lib64 /lib64 \
      --ro-bind-try /usr/lib /usr/lib \
      --ro-bind-try /usr/lib64 /usr/lib64 \
      --ro-bind-try /usr/libexec /usr/libexec \
      --bind "$cwd" "$cwd" \
      --dev-bind /dev/null /dev/null \
      --dev-bind /dev/zero /dev/zero \
      --dev-bind /dev/random /dev/random \
      --unshare-net \
      --unshare-ipc \
      --cap-drop ALL \
      --chdir "$cwd" \
      "$@"

[3] https://github.com/containers/bubblewrap