cant use apt-update anymore

I realized that https://download.docker.com/dists/jessie/main/binary-armhf/Packages (the URL repository your computer has) returned me an error 404 as well, and after a bit of searching found out https://download.docker.com/linux/debian/dists/jessie/stable/binary-armhf/ resolves well. So my opinion is that Docker has moved its repositories to some other directory. This can be solved by editing the URL to fetch the packages.

Try adding this line on /etc/apt/sources.list:

deb https://download.docker.com/linux/debian/dists/jessie/stable/binary-armhf jessie devel admin

and comment the line which should look something like this:

deb https://download.docker.com/dists/jessie/stable/binary-armhf jessie devel admin

(note the second line doesn't contain linux/debian/)