docker: command not found in gitlab-ci - docker

Question

Background
in my gitlab-ci file I am trying to build a docker image, however even though I have docker:dind as a service, it is failing.
.gitlab-ci
---
stages:
- build
- docker
build:
stage: build
image: fl4m3ph03n1x/my-app:1.0
variables:
MIX_ENV: prod
script:
- mix deps.get
- mix deps.compile
- mix compile
artifacts:
paths:
- .hex/
- _build/
- deps/
- mix.lock
build_image:
stage: docker
image: fl4m3ph03n1x/my-app:1.0
variables:
DOCKER_DRIVER: overlay2
DOCKER_TLS_CERTDIR: ""
DOCKER_HOST: tcp://docker:2375/
services:
- docker:dind
script:
- echo ${CI_JOB_TOKEN} | docker login --password-stdin -u ${CI_REGISTRY_USER} ${CI_REGISTRY}
- docker build . -t ${CI_REGISTRY_IMAGE}:latest
- docker push ${CI_REGISTRY_IMAGE}:latest
The problematic stage is docker.
As you can see I am trying to:
login into docker
build an image from gitlab's registry
push that image
Error
However, I am getting the following error:
$ echo ${CI_JOB_TOKEN} | docker login --password-stdin -u
${CI_REGISTRY_USER} ${CI_REGISTRY} /bin/bash: line 110: docker:
command not found
Which is confusing, because docker:dind is supposed to actually prevent this from happening:
https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#enable-registry-mirror-for-dockerdind-service
Question
So clearly I am missing something here. What am I doing wrong?
EDIT
This is my Dockerfile
FROM elixir:1.10
# Install Hex + Rebar
RUN mix do local.hex --force, local.rebar --force
COPY . /
WORKDIR /
ENV MIX_ENV=prod
RUN mix do deps.get --only $MIX_ENV, deps.compile
RUN mix release
EXPOSE 8080
ENV PORT=8080
ENV SHELL=/bin/bash
CMD ["_build/prod/rel/my_app/bin/my_app", "start"]

Answer 1

image is used to specify the image in which to run the script. You want to run the script in a docker image, to build your image.
The image keyword is the name of the Docker image the Docker executor runs to perform the CI tasks.
https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#define-image-and-services-from-gitlab-ciyml
After all, isn't your application image CI_REGISTRY_IMAGE in this? You don't want to build the image in itself.
- docker build . -t ${CI_REGISTRY_IMAGE}:latest
- docker push ${CI_REGISTRY_IMAGE}:latest