Build stuck at fetching docker image "python:2.7"

Summary

My build is stick at Pulling docker image python:2.7 ...

Steps to reproduce

I'm not sure how reproducible this is. My project is private, but here's the .gitlab-ci.yaml file:

image: "python:2.7"
stages:
  - test
  - deploy
backend_unit_tests:
  script:
    - pip install -r requirements.txt
    - py.test
  stage: test
deploy_production:
  environment: production
  script:
    - git remote add heroku $HEROKU_PUSH_URL
    - git push heroku master
  stage: deploy
  only:
  - master
pages:
  stage: deploy
  script:
  - cp -r landingpage public
  artifacts:
    paths:
    - public
  only:
  - master
variables:
  SECRET_KEY: "testing"
  DATABASE_URL: "psql://test/test"
  DEBUG: "true"
  SENDGRID_USERNAME: "FOO"
  SENDGRID_PASSWORD: "BAR"

The job that's stuck is backend_unit_tests.

The project itself is private, but maybe, if this is useful, someone with access to the server can get the build by its id: 5303640. The id's don't look like they're per-project.

Expected behavior

The build goes from pending to running, and passes/fails

Actual behavior

The build is stuck

Relevant logs and/or screenshots

N/A