If we wish to increase the capabilities of our operating system, we have the option of adding repositories to be able to install updated software or software that is not available for our distribution.
It is important to mention that it is necessary to investigate the reliability of a repository in order to avoid security problems on our server.
The file "/etc/apt/sources.list" contains the list of repositories currently on the system. For example, in Debian10 we find:
deb http://deb.debian.org/debian/ buster main deb-src http://deb.debian.org/debian/ buster main deb http://security.debian.org/debian-security buster/updates main deb-src http://security.debian.org/debian-security buster/updates main deb http://deb.debian.org/debian/ buster-updates main deb-src http://deb.debian.org/debian/ buster-updates main
If we wanted, for example, to add the official Docker repository for Debian10, we should add the following line in the file:
deb https://download.docker.com/linux/debian buster stable
And this way, we would have added the docker repository.