title: GitLab CI invalid argument on job for Docker build
body: <p>So I'm trying to setup my Gitlab CI to trigger a job on <code>git push</code> to build and deploy my Docker. This is the <code>.gitlab-ci.yml</code> file I'm using based on an example from Gitlab docs (Elixir yml).</p>
  
  <pre><code>stages:
    - build
  
  build:
    before_script:
      - docker build -f Dockerfile.build -t ci-project-build-$CI_PROJECT_ID:$CI_BUILD_REF .
      - docker create
        -v /build/deps 
        -v /build/_build
        -v /build/rel
        -v /root/.cache/aceapp/
        --name build_data_$CI_PROJECT_ID_$CI_BUILD_REF busybox /bin/true
    tags:
      - docker
    stage: build
    script: 
      - docker run --volumes-from build_data_$CI_PROJECT_ID_$CI_BUILD_REF --rm -t ci-project-build-$CI_PROJECT_ID:$CI_BUILD_REF
  </code></pre>
  
  <p>The output when pushing to GitLab instance is this:</p>
  
  <pre><code>Running with gitlab-runner 10.7.2 (b5e03c94)
  on my.host.rhel.runner 8f724ea7
  Using Shell executor...
  Running on my.host.local...
  Fetching changes...
  HEAD is now at 14351c4 Merge branch 'Development' into 'master'
  From https://my.host.example/zalmosc/ace-app
     14351c4..9fa2d43  master     -&gt; origin/master
  Checking out 9fa2d435 as master...
  Skipping Git submodules setup
  $ # Auto DevOps variables and functions # collapsed multi-line command
  $ setup_docker
  $ build
  Logging to GitLab Container Registry with CI credentials...
  Login Succeeded
  
  Building Dockerfile-based application...
  invalid argument "/master:9fa2d4358e6c426b882e2251aa5a49880013614b" for t: Error parsing reference: "/master:9fa2d4358e6c426b882e2251aa5a49880013614b" is not a valid repository/tag: invalid reference format
  See 'docker build --help'.
  ERROR: Job failed: exit status 1
  </code></pre>
  
  <p>I understand the docker tag is not valid (is the <code>before_script:</code> really triggered based on the name?), and I'm looking for help regarding a) a solution b) how I can learn more about the requirements for a pipeline that builds docker based on default settings. Do I need to tag my docker image locally and then somehow add this to my git commit? </p>