Passing environment variable from workflow to job

I am using circle ci to build couple of docker images. The docker image rely on -build-arg my_var=val. I have modified my circleci job to handle this arg i.e.,

  build_image:
    docker: 
      - image: google/cloud-sdk:latestare run
    steps:
      - run: docker build --build-arg my_var=${my_var} -t .

Now the problem is that how would i pass my_var from workflow to the job. I used command but i think its illegal there.

workflows:
  version: 2

  build-image-and-deploy:
    jobs:
      -  build_image:
         command: |
            echo "export my_var=val" >> $BASH_ENV