apt-get update shows errors

Question

I would not have written had I but found one similar problem solved over the internet. I have searched almost all Q&A sites and am stuck at the exact same spot as 4 hours ago. I followed the documentation for installing Docker CE on my Ubuntu 17.10 system to the letter. However, when I try running sudo apt-get update, I get the following error on my console:

    Ign:1 http://ppa.launchpad.net/mc3man/xerus-media/ubuntu artful InRelease      
Get:2 http://security.ubuntu.com/ubuntu artful-security InRelease [78.6 kB]    
Hit:3 https://download.docker.com/linux/ubuntu artful InRelease                
Hit:4 http://in.archive.ubuntu.com/ubuntu artful InRelease                     
Err:5 http://ppa.launchpad.net/mc3man/xerus-media/ubuntu artful Release        
  404  Not Found
Get:6 http://in.archive.ubuntu.com/ubuntu artful-updates InRelease [78.6 kB]   
Get:7 http://in.archive.ubuntu.com/ubuntu artful-backports InRelease [72.2 kB]
Reading package lists... Done      
E: The repository 'http://ppa.launchpad.net/mc3man/xerus-media/ubuntu artful Release' does not 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.

And needless to say, I cannot proceed with Docker. How do I fix this? P.S., I tried editing my sources.list file, but it was ReadOnly and I couldn't save the changes. As an added fact, I noticed that at http://ppa.launchpad.net/mc3man/xerus-media/ubuntu/ , the Release files are located inside the dist/Xenial folder.

Please help me fix this problem.

Edit: Here is my sources.list file for added information.

# deb cdrom:[Ubuntu 17.10 _Artful Aardvark_ - Release amd64 (20171018)]/ artful main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://in.archive.ubuntu.com/ubuntu/ artful main restricted
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://in.archive.ubuntu.com/ubuntu/ artful-updates main restricted
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ artful universe
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful universe
deb http://in.archive.ubuntu.com/ubuntu/ artful-updates universe
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://in.archive.ubuntu.com/ubuntu/ artful multiverse
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful multiverse
deb http://in.archive.ubuntu.com/ubuntu/ artful-updates multiverse
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://in.archive.ubuntu.com/ubuntu/ artful-backports main restricted universe multiverse
# deb-src http://in.archive.ubuntu.com/ubuntu/ artful-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu artful partner
# deb-src http://archive.canonical.com/ubuntu artful partner

deb http://security.ubuntu.com/ubuntu artful-security main restricted
# deb-src http://security.ubuntu.com/ubuntu artful-security main restricted
deb http://security.ubuntu.com/ubuntu artful-security universe
# deb-src http://security.ubuntu.com/ubuntu artful-security universe
deb http://security.ubuntu.com/ubuntu artful-security multiverse
# deb-src http://security.ubuntu.com/ubuntu artful-security multiverse
deb [arch=amd64] https://download.docker.com/linux/ubuntu artful stable
# deb-src [arch=amd64] https://download.docker.com/linux/ubuntu artful stable
# deb-src [arch=amd64] https://download.docker.com/linux/ubuntu artful stable
# deb-src [arch=amd64] https://download.docker.com/linux/ubuntu artful stable
# deb-src [arch=amd64] https://download.docker.com/linux/ubuntu artful stable

Answer 1

You have two issues conflated, for starters lets work on xerus-media

cd /etc/apt
grep -r xerus *

now edit the file containing xerus

sudo vi /etc/apt/either_sources.list_or_a_file_in_subdir

delete the line which mentions

http://ppa.launchpad.net/mc3man/xerus-media/ubuntu artful Release   

add these lines

deb http://ppa.launchpad.net/mc3man/xerus-media/ubuntu xenial main 
deb-src http://ppa.launchpad.net/mc3man/xerus-media/ubuntu xenial main      

save that file and confirm you no longer see an error which mentions xerus-media

sudo apt-get update 

Now on to the docker issue ... docker package for ubuntu is shipped to xenial ... so lets see what you have ... do this

cd /etc/apt
grep -r docker *

now edit the file which mentions docker as in

sudo vi /etc/apt/sources.list

now edit the line

deb [arch=amd64] https://download.docker.com/linux/ubuntu artful stable

its entry should look like this

deb [arch=amd64] https://download.docker.com/linux/ubuntu xenial stable

(yes xenial its OK its not artful you still get newest docker) now lets see what docker package you have installed ... issue this

dpkg -l | grep docker

you should see this

ii  docker-ce       17.09.0~ce-0~ubuntu      amd64        Docker: the open-source application container engine

now do an update

sudo apt-get update && sudo apt-get dist-upgrade 

are you now locked and loaded ? if not we can continue battling