Trouble connecting to Docker daemon in GitLab CI - docker

Question

I'm trying to create a fairly simple GitLab CI file to build out Docker images. Whenever I run the pipeline, I end up getting a Docker daemon connection issue. What can I do to properly build my image? Thanks!
GitLab CI:
image: docker:20.10.16
services:
- docker:20.10.16-dind
variables:
DOCKER_HOST: tcp://docker:2375
iac-build:
stage: build
extends: .iac
rules:
- if: $CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH
when: always
- if: $CI_PIPELINE_SOURCE == 'merge_request_event'
when: never
- if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
when: always
script:
- docker build -t testfirstimage .
allow_failure: false
Error:
$ docker build -t testfirstimage .
failed to dial gRPC: cannot connect to the Docker daemon. Is 'docker daemon' running on this host?: dial tcp 127.0.0.1:2375: connect: connection refused
Cleaning up project directory and file based variables
ERROR: Job failed: command terminated with exit code 1

Answer 1

First thing I would check is if you already have something running on that local host - I've literally tried running a server on a local host port for hours and kept having it refused, only to find out that I had forgotten to terminate my connection to that port.
If that isn't the issue, I had this issue before and had to run this command to get it to work:
concurrent = 1
check_interval = 0
[[runners]]
name = "#####"
url = "#####"
token = "#####"
executor = "docker"
[runners.docker]
tls_verify = false
image = "docker:latest"
privileged = false
disable_cache = false
cache_dir = "cache"
volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
[runners.cache]
Insecure = false
I spent forever trying to figure it out and couldn't get anything to work until I found out to add
volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
I didn't figure it out by magic though - props to this issues page: https://gitlab.com/gitlab-org/gitlab-runner/-/issues/1986
Hopefully that works.

Answer 2

In order to get Docker-in-Docker working with GitLab CI, you will first need to decide if you want to use Docker-in-Docker with or without TLS. Then, change /etc/gitlab-runner/config.toml settings, and assign the DOCKER_TLS_CERTDIR in your .gitlab-ci.yml file. See the Docker-in-docker section of the GitLab docs.
Docker-in-docker with TLS:
# /etc/gitlab-runner/config.toml
[[runners]]
url = "https://gitlab.com/"
token = TOKEN
executor = "docker"
[runners.docker]
tls_verify = false
image = "docker:20.10.16"
privileged = true
disable_cache = false
volumes = ["/certs/client", "/cache"]
[runners.cache]
[runners.cache.s3]
[runners.cache.gcs]
# .gitlab-ci.yml
image: docker:20.10.16
variables:
DOCKER_TLS_CERTDIR: "/certs"
services:
- docker:20.10.16-dind
before_script:
- docker info
# rest of .gitlab-ci.yml