apt-get update Fails

[jjdd12]

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

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.```