You should check your networking for proxies or other issues.
If you need to you can manually copy the files into the correct location.
root@0c988f5d8e4c:/# cat /etc/ros/rosdep/sources.list.d/20-default.list
# os-specific listings first
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
# generic
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml
gbpdistro https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml fuerte
# newer distributions (Groovy, Hydro, ...) must not be listed anymore, they are being fetched from the rosdistro index.yaml instead
You will also get this error if your ssl certificates are unavailable.
root@0c988f5d8e4c:/# apt remove ca-certificates
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
file libexpat1 libexpat1-dev libffi6 libmagic1 libpython-all-dev libpython-dev libpython-stdlib libpython2.7 libpython2.7-dev
libpython2.7-minimal libpython2.7-stdlib libsqlite3-0 libssl1.0.0 mime-support openssl python python-all python-all-dev python-dev
python-minimal python-pkg-resources python-setuptools python-wheel python2.7 python2.7-dev python2.7-minimal
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
ca-certificates python-pip python-pip-whl
0 upgraded, 0 newly installed, 3 to remove and 35 not upgraded.
After this operation, 2351 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 16375 files and directories currently installed.)
Removing python-pip (8.1.1-2ubuntu0.4) ...
Removing python-pip-whl (8.1.1-2ubuntu0.4) ...
Removing ca-certificates (20160104ubuntu1) ...
Removing dangling symlinks from /etc/ssl/certs... done.
root@0c988f5d8e4c:/# rosdep init
ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.