How can I solve the errors in apt update in raspberry pi? - raspberry-pi

Question

Recently I edit my /etc/apt/sources.list and change jessie to stretch to get the new version of gcc-6. However I when I change stretch back to jessie, and run sudo apt-get update I got a lot of errors. How can I solve it.
pi#raspberrypi ~ $ sudo apt update
Hit:1 http://raspberrypi.collabora.com wheezy InRelease
Get:2 http://mirrordirector.raspbian.org/raspbian jessie InRelease [14.9 kB]
Err:1 http://raspberrypi.collabora.com wheezy InRelease
The following signatures were invalid: FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
Hit:4 http://archive.raspberrypi.org/debian jessie InRelease
Get:5 http://mirrordirector.raspbian.org/raspbian jessie/main armhf Packages [9534 kB]
Hit:3 https://packagecloud.io/Hypriot/Schatzkiste/raspbian jessie InRelease
Get:6 http://124.202.164.8/files/311600000A34E137/http.debian.net/debian jessie/main armhf Packages [169 kB]
Err:6 http://124.202.164.8/files/311600000A34E137/http.debian.net/debian jessie/main armhf Packages
Writing more data than expected (196868 > 169365)
Hashes of expected file:
- Filesize:169365 [weak]
- SHA256:779179f25b91efa4fdbf69aeb7c37d444e104e3b8d359daf27bc4db4d330003f
- SHA1:aa605ce914ddacfd4b8c71c72bab3a95b76c0ed5 [weak]
- MD5Sum:f0867f34045767904d0caa686f288615 [weak]
Release file created at: Fri, 14 Jul 2017 15:58:01 +0000
Get:7 http://archive.raspbian.org/raspbian jessie InRelease [14.9 kB]
Get:8 http://archive.raspbian.org/raspbian jessie/rpi Sources [1132 B]
Get:9 http://archive.raspbian.org/raspbian jessie/non-free Sources [113 kB]
Get:10 http://archive.raspbian.org/raspbian jessie/main Sources [7754 kB]
Ign:11 https://apt.dockerproject.org/repo raspbian-jessie InRelease
Get:12 http://mirrordirector.raspbian.org/raspbian jessie/contrib armhf Packages [43.3 kB]
Get:13 http://mirrordirector.raspbian.org/raspbian jessie/non-free armhf Packages [84.2 kB]
Get:14 http://mirrordirector.raspbian.org/raspbian jessie/rpi armhf Packages [1356 B]
Err:15 https://apt.dockerproject.org/repo raspbian-jessie Release
gnutls_handshake() failed: The TLS connection was non-properly terminated.
Get:16 http://archive.raspbian.org/raspbian jessie/contrib Sources [64.5 kB]
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://raspberrypi.collabora.com wheezy InRelease: The following signatures were invalid: FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
E: The repository 'https://apt.dockerproject.org/repo raspbian-jessie Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
My sources.list file
deb http://mirrordirector.raspbian.org/raspbian/ jessie main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
deb-src http://archive.raspbian.org/raspbian/ jessie main contrib non-free rpi

Answer 1

I had the same issue and below solution worked for me.
Create a file called badproxy with below lines:
Acquire::http::Pipeline-Depth 0;
Acquire::http::No-Cache true;
Acquire::BrokenProxy true;
then copy the file into the apt config folder:
cp ./badproxy /etc/apt/apt.conf.d/99fixbadproxy
I tried above suggestion from docker form

Answer 2

sudo apt remove gnupg
sudo apt install --reinstall gnupg2 (possibly not necessary, but doesn't hurt)
sudo apt install dirmngr
Then run
gpg --keyserver keys.gnupg.net --recv-key FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5
gpg -a --export FF9FD9E73808E2376CB76F0AED4BF9140C50B1C5 | sudo apt-key add -