The build job failed with:
Cannot connect to the Docker daemon at tcp://localhost:2375.
Is the docker daemon running?
Three things were wrong:
1. DOCKER_HOST was set to tcp://localhost:2375. The DinD daemon runs in
the service sidecar container, not in the build container, so
localhost was always going to refuse the connection. The correct host
is the service alias 'docker'.
2. The docker:28.4.0-dind service was declared without an explicit
alias. Without alias: docker, GitLab derives the hostname from the
image string 'docker:28.4.0-dind', and since ':' is invalid in DNS,
the 'docker' hostname never resolves. The explicit alias is required
for tcp://docker:2375 to work at all.
3. docker:28.4.0-dind enables TLS by default and listens on 2376, but
DOCKER_HOST points at 2375. Setting DOCKER_TLS_CERTDIR="" disables
TLS so the daemon listens on plaintext 2375, matching DOCKER_HOST.
This mirrors the known-working pattern in the vnc-localidp pipeline
(docker:20.10.17-dind + alias: docker + DOCKER_HOST=tcp://docker:2375
+ DOCKER_TLS_CERTDIR=""). The TLS-defaults behavior has been unchanged
since docker 19.03, so the same pattern applies on 28.4.0.
DOCKER_TLS_CERTDIR forced the docker:28.4.0-dind service to listen on
port 2376 with TLS, but DOCKER_HOST pointed to the non-TLS port 2375.
This caused the docker client to loop forever with:
Cannot connect to the Docker daemon at tcp://localhost:2375.
Removing DOCKER_TLS_CERTDIR lets the daemon listen on 2375 again,
matching DOCKER_HOST, restoring docker-in-docker connectivity.
Stefan-Sanger