documentation: update instructions on how to install Docker

The old instructions were for Debian 8 ("jessie") which has
reached its end of life a while ago, so they are replaced with
instructions for a newer version, in this case Debian 11
(codename "bullseye"). These instructions should also work for
the current stable release Debian 12 (codename "bookworm").

[ci skip]

documentation/docker.md

@@ -4,16 +4,10 @@
 
 If you prefer the non-native approach and like to use Docker instead, you have
 to make sure that Docker is installed. For the purpose of this installation,
-we will assume that you are using Debian 8.0 "jessie". The required steps should
-be similar on other Linux distributions.
+we will assume that you are using Debian 11 (codename "bullseye"). The required
+steps should be similar on other Linux distributions.
 
-To install Docker, you need to add the APT repository for jessie-backports to
-your APT sources, if that has not been done yet. Execute the following command
-as root user to add the AP repository:
-
- echo "deb http://ftp.de.debian.org/debian/ jessie-backports main" >> /etc/apt/sources.list
-
-After that you can install Docker by typing the following commands:
+On a Debian 11 system you can install Docker by typing the following commands:
 
  # update of package lists
  apt-get update
@@ -33,10 +27,6 @@ users might need to log off and on again before they can continue.
 
 Furthermore you should restart the Docker daemon:
 
- systemctl start docker
-
-or
-
  systemctl restart docker
 
 should trigger the restart.
@@ -47,7 +37,7 @@ After that the user _user1_ can type
 
 into a terminal in order to check whether he/she can execute docker commands in
 his/her user context. If the command displays an error, then something is not
-quite right yet.
+quite right yet. (Forgot to log off and on again?)
 
 ## Let's party: Build docker image and create container