Is dockerfile of jupyter-notebook in example newest?

Hello,

I am trying to build jupyter-notebook image based on Dockerfile.cpu in example directory:

https://github.com/GoogleCloudPlatform/container-engine-accelerators/blob/4baa941d8df91e5dc1736adea74ce54e564dd782/example/tensorflow-notebook-image/Dockerfile.cpu#L3

but got following error:

```console
Package libav-tools is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
ffmpeg

E: Package 'libav-tools' has no installation candidate
The command '/bin/sh -c apt-get update && apt-get install -yq --no-install-recommends apt-transport-https build-essential bzip2 ca-certificates curl emacs fonts-liberation g++ git inkscape jed libav-tools libcupti-dev libsm6 libxext-dev libxrender1 lmodern locales lsb-release openssh-client pandoc pkg-config python python-dev sudo unzip vim wget zip zlib1g-dev && apt-get clean && rm -rf /var/lib/apt/lists/*' returned a non-zero code: 100
```

After change `libav-tools` to `ffmpeg` will fix this error.So is the dockerfile newest or not?