Add basic Docker configuration

# * `.gitlab-ci.yml` file copied and modified from the GitLab Documentation (as of 2021-01-10, the contents of the script seem to have changed by now)

#   * Title of the documentation page: "Building a Docker image with kaniko"

#   * Author: (c) 2011-present GitLab B.V.

#   * URL: https://docs.gitlab.com/ee/ci/docker/using_kaniko.html#building-a-docker-image-with-kaniko

#   * License: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)

docker-build:

    image:

        name: gcr.io/kaniko-project/executor:debug

        entrypoint: [""]

    stage: build

    before_script:

        - mkdir -p /kaniko/.docker

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

    script:

        - echo "{\"commit\":\"$CI_COMMIT_SHA\",\"datetime\":\"$CI_COMMIT_TIMESTAMP\"}" > PORTAL_VERSION

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

    only:

        - master

FROM python:latest