Summon Docker build error

[jtuttle]

I was trying to run the demo and getting this error when trying to build the summon image in step 5:

Step 3/5 : RUN apt-get update &&     apt-get install -y --no-install-recommends curl
 ---> Running in 5b58c62fb2aa
Ign http://deb.debian.org jessie InRelease
Get:1 http://security.debian.org jessie/updates InRelease [44.9 kB]
Get:2 http://deb.debian.org jessie-updates InRelease [7340 B]
Get:3 http://deb.debian.org jessie Release.gpg [2420 B]
Get:4 http://deb.debian.org jessie Release [148 kB]
Get:5 http://security.debian.org jessie/updates/main amd64 Packages [865 kB]
Get:6 http://deb.debian.org jessie/main amd64 Packages [9098 kB]
Fetched 10.2 MB in 6s (1508 kB/s)
W: Failed to fetch http://deb.debian.org/debian/dists/jessie-updates/InRelease  Unable to find expected entry 'main/binary-amd64/Packages' in Release file (Wrong sources.list entry or malformed file)

E: Some index files failed to download. They have been ignored, or old ones used instead.
The command '/bin/sh -c apt-get update &&     apt-get install -y --no-install-recommends curl' returned a non-zero code: 100

I was able to fix it by following this solution but this seems like an odd thing to have to add to our Dockerfile. It would like to better understand the error and confirm if it's happening for other people before committing a fix.