Define `.build-docker-image` template used by `*-docker-build` jobs It
build the container image and push it to `${CI_REGISTRY}`

USAGE
=====

foo-docker-build:
  extends: .build-docker-image
  variables:
    DOCKERFILE: Some-specific.Dockerfile
    IMAGE_TAG: $CI_COMMIT_REF_SLUG

REQUIREMENTS
============

- A `build` stage must be present in your pipeline or it must be
  overriden by the extending job to feet your need.
- The `.not-on-stable` rules template

OPTIONAL VARIABLES
==================

- `IMAGE_NAME`: defaults to `${CI_JOB_NAME}` with any suffix
  `-docker-build*` removed
- `DOCKERFILE`: defaults, in order
   - to value set by the user
   - to `Dockerfile.${IMAGE_NAME}` if the file exists
   - to `Dockerfile` if the file exists
- `IMAGE_TAG`: defaults to `git-${CI_COMMIT_SHORT_SHA}`

USED CI VARIABLES
=================

- `CI_REGISTRY`
- `CI_REGISTRY_USER`
- `CI_REGISTRY_PASSWORD`
- `CI_REGISTRY_IMAGE`