How to pass variables from .gitlab.ci.yml to Dockerfile

How can I pass one variables value from .gitlab-ci.yml to Dockerfile?

e.g. The .gitlab-ci.yml contains:

variables:
  var1: ex_variable_1
  var2: ex_variable_2

stages:
  - build

build:
  stage: build
  script:
    - sudo docker build . -t ${CI_PROJECT_PATH_SLUG}
                          --build-arg var1
                          --build-arg var2
    - sudo docker run -dit --name ${CI_PROJECT_PATH_SLUG} --cap-add=NET_ADMIN ${CI_PROJECT_PATH_SLUG}:latest

The Dockefile contains:

FROM centos:6.9

ENV var1 ${var1}
ENV var2 ${var2}

RUN echo "Print var1 $var1"
RUN echo "Print var2 $var2

So what I want is to pass var1 and var2 from .gitlab-ci.yml to Dockerfile.

To pass environment variables from .gitlab-ci.yml to an image or container:

• at build time (if the variable is used by the Dockerfile):

  • use the docker build --build-arg option: docker build --build-arg var1="$var1"

  • and change your Dockerfile like this:

      FROM centos:6.9
      ARG var1
      ARG var2
    
      RUN echo "Print var1 $var1"
      RUN echo "Print var2 $var2"
    
      # The following is optional − only useful if you want to
      # keep the environment variables at runtime (docker run)
      ENV var1=${var1}
      ENV var2=${var2}
    

    Note: the ARG instruction can also take a default value:

      ARG var1="default value if var1 was not passed using --build-arg"
    

• at runtime (if the variable is used by the container, i.e., by the ENTRYPOINT / CMD-specified program):

  • use the docker run -e CLI option: docker run -e var1="$var1".