Docker and its Docker.io debian pakcage is currently not available Debian Jessie thus:
 Package 'docker.io' has no installation candidate
The easiest way to install docker is to use get.docker.com script: First, install curl tool:
# apt-get install curl
Once ready download and run get.docker.com script:
# curl -sSL https://get.docker.com/ | sh
+ sh -c sleep 3; apt-get update
Get:1 http://debian.mirror.uber.com.au jessie InRelease [214 kB]
...
Processing triggers for libc-bin (2.19-13) ...
+ sh -c docker version
Client version: 1.6.0
Client API version: 1.18
Go version (client): go1.4.2
Git commit (client): 4749651
OS/Arch (client): linux/amd64
Server version: 1.6.0
Server API version: 1.18
Go version (server): go1.4.2
Git commit (server): 4749651
OS/Arch (server): linux/amd64

If you would like to use Docker as a non-root user, you should now consider
adding your user to the "docker" group with something like:

  sudo usermod -aG docker your-user

Remember that you will have to log out and back in for this to take effect!
Test your docker installation by creating a Ubuntu docker container:
# docker run -i -t ubuntu /bin/bash
Unable to find image 'ubuntu:latest' locally

latest: Pulling from ubuntu
511136ea3c5a: Pull complete 
f3c84ac3a053: Pull complete 
a1a958a24818: Pull complete 
9fec74352904: Pull complete 
d0955f21bf24: Already exists 
Digest: sha256:7b27f7cc97d4c94fdebb2cf99ddaadd0e8fc8ec4aed2cf56a8ee8fe7dc2ec4a4
Status: Downloaded newer image for ubuntu:latest

root@f3e5b3b575bd:/# cat /etc/issue
Ubuntu 14.04.2 LTS \n \l