Hello Louis,
I suppose you started build agent inside a docker image. To get Docker support working, you should start teamcity build agent with docker-in-docker support. Please find “Docker in Docker support” section on the page https://hub.docker.com/r/jetbrains/teamcity-agent/
If you started teamcity agent manually on a simple Linux machine, you should install docker on this machine first.
This should be something like
$ sudo dnf install docker
$ sudo systemctl start docker
– before starting the build agent of TeamCity.
Hope this helps,