Automatically build and push on update

Use GitLab CI to build and push images to the registry on update, with support for multiple Dockerfiles in one repo, using directory structure as tag names.

The Docker .gitlab-ci.yml template:

# This file is a template, and might need editing before it works on your project.
# Official docker image.
image: docker:latest

services:
  - docker:dind

before_script:
  - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY

build-master:
  stage: build
  script:
    - docker build --pull -t "$CI_REGISTRY_IMAGE" .
    - docker push "$CI_REGISTRY_IMAGE"
  only:
    - master

build:
  stage: build
  script:
    - docker build --pull -t "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG" .
    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG"
  except:
    - master