add docker build stage

stages:

  - build

docker:

  stage: build

  image:

    name: gcr.io/kaniko-project/executor:v0.20.0

    entrypoint: [""]

  script:

    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json

    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG --destination $CI_REGISTRY_IMAGE:latest

  only:

    - tags