[build] ERROR: Sonar server 'http://localhost:9000' can not be reached

If you use SonarScanner CLI with Docker, you may have this error because the SonarScanner container can not access to the Sonar UI container.

Note that you will have the same error with a simple curl from another container:

docker run --rm byrnedo/alpine-curl 127.0.0.1:9000
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (7) Failed to connect to 127.0.0.1 port 8080: Connection refused

The solution is to connect the SonarScanner container to the same docker network of your sonar instance, for instance with --network=host:

docker run --network=host -e SONAR_HOST_URL='http://127.0.0.1:9000' --user="$(id -u):$(id -g)" -v "$PWD:/usr/src" sonarsource/sonar-scanner-cli

(other parameters of this command comes from the SonarScanner CLI documentation)