apt-get update Fails

this is the resulting error
`W: There is no public key available for the following key IDs:
9D6D8F6BC857C906`

```Removing intermediate container 479c0d086d3b

Step 2/16 : RUN apt-get update && apt-get install -y --no-install-recommends build-essential gettext git libffi-dev libjpeg-dev libmemcached-dev libpq-dev libssl-dev libxml2-dev libxslt1-dev locales nginx python-dev python-virtualenv python3-dev sudo supervisor zlib1g-dev && apt-get clean && rm -rf /var/lib/apt/lists/* && dpkg-reconfigure locales && locale-gen C.UTF-8 && /usr/sbin/update-locale LANG=C.UTF-8 && mkdir /etc/pretix && mkdir /data && useradd -ms /bin/bash -d /pretix -u 15371 pretixuser && echo 'pretixuser ALL=(ALL) NOPASSWD: /usr/bin/supervisord' >> /etc/sudoers && mkdir /static

---> Running in ce9daa6fe1d6

Get:1 http://security.debian.org jessie/updates InRelease [44.9 kB]

Ign http://http.debian.net jessie InRelease

Ign http://http.debian.net jessie-updates InRelease

Get:2 http://http.debian.net jessie Release.gpg [2420 B]

Ign http://http.debian.net jessie-updates Release.gpg

Get:3 http://http.debian.net jessie Release [148 kB]

Ign http://http.debian.net jessie-updates Release

Get:4 http://security.debian.org jessie/updates/main amd64 Packages [825 kB]

Get:5 http://http.debian.net jessie/main amd64 Packages [9098 kB]

Err http://http.debian.net jessie-updates/main amd64 Packages

Err http://http.debian.net jessie-updates/main amd64 Packages

Err http://http.debian.net jessie-updates/main amd64 Packages

Err http://http.debian.net jessie-updates/main amd64 Packages
404 Not Found

Fetched 10.1 MB in 8s (1190 kB/s)

W: There is no public key available for the following key IDs:
9D6D8F6BC857C906
W: Failed to fetch http://http.debian.net/debian/dists/jessie-updates/main/binary-amd64/Packages 404 Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.```

Start by locating the Dockerfile containing the shown Step 2/16 RUN apt-get update command and inspect its Debian jessie package sources. Reproduce the image build, then verify that apt-get update and installation of the listed packages complete without the missing-key or 404 errors.

索引モデルが issue の本文から書いたものです。