Example release stage

.travis.yml

dist: xenial
services:
  - docker
language: bash
env:
  global:
    - IMAGE_TAG=praekeltorg/mysite
    - REGISTRY_USER=praekeltorg-automation
    - secret: <REGISTRY_PASS encrypted>

before_script:
  - docker pull "$IMAGE_TAG" || true
script:
  - docker build --pull --cache-from "$IMAGE_TAG" -t "$IMAGE_TAG" .

before_deploy:
  - echo -n "$REGISTRY_PASS" | docker login -u "$REGISTRY_USER" --password-stdin
deploy:
  provider: script
  script: docker push "$IMAGE_TAG"
  on:
    branch: master