Where does the string docker.example.com in my GitLab CI job come from?

I am trying to have a Docker image deployed to my server by GitLab CI, but it only worked once¹ and then dies with the error message below. Here is the .gitlab-ci.yml snippet:

deploy-image:
  image: docker:latest
  services:
    - docker:dind
  variables:
    DOCKER_HOST: ssh://gitlab-ci@$DEPLOY_HOST
  before_script:
    - eval $(ssh-agent -s)
    - echo "${SSH_KEY}" | ssh-add -
    - mkdir -p ~/.ssh
    - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
  script:
    - echo $CI_JOB_TOKEN | docker login --username gitlab-ci-token --password-stdin $CI_REGISTRY;
    - docker pull "$CI_REGISTRY_IMAGE"
    - docker run --name mycontainer --detach $CI_REGISTRY_IMAGE
  after_script:
    - docker logout $CI_REGISTRY

The error in the job log looks like that:

Executing "step_script" stage of the job script 00:04
Using docker image sha256:51453dcdd9bd51e503f75e6d42a4071469ad2ba816321781985041b6bc7776db for docker:latest with digest docker@sha256:ddf0d732dcbc3e2087836e06e50cc97e21bfb002a49c7d0fe767f6c31e01d65f ...
$ eval $(ssh-agent -s)
Agent pid 18
$ echo "${SSH_KEY}" | ssh-add -
Identity added: (stdin) (gitlab-ci@jacks)
$ mkdir -p ~/.ssh
$ [[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config
$ echo $CI_JOB_TOKEN | docker login --username gitlab-ci-token --password-stdin $CI_REGISTRY;
error during connect: Post "http://docker.example.com/v1.24/auth": command [ssh -l gitlab-ci -- xxx.xxx.xxx.xxx docker system dial-stdio] has exited with exit status 255, please make sure the URL is valid, and Docker 18.09 or later is installed on the remote host: stderr=ssh: connect to host xxx.xxx.xxx.xxx port 22: Connection refused

While I x-ed out the IP address of the destination server in this post (which is the value of the DEPLOY_HOST env var), it literally says docker.example.com in the error message.

So my question would be, where that example string comes from.

This is on a self-hos

It's a placeholder domain name for connecting over SSH. It doesn't mean that it's connecting to docker.example.com.

I ran into a similar error when using a Docker client over SSH. The error message was:

ERROR: Cannot connect to the Docker daemon at http://docker.example.com. Is the docker daemon running?

Reading the Docker source code, it looks like "docker.example.com" is a placeholder host name when connecting over SSH, resulting in the confusing error message.

In my case, the error was caused by the user I was logging in as not being part of the docker group, and therefore not having permission to use docker.

StackExchange DevOps Q#14601, answer score: 10