feat: build docker stages

Closes #78 (closed)

This MR adds the ability to build and publish specified stages from the Dockerfile by including /auto-devops/multi-target-docker-images.yml alongside other templates:

include:
  - project: "uis/devops/continuous-delivery/ci-templates"
    file: "/auto-devops/multi-target-docker-images.gitlab-ci.yml"
    inputs:
      docker_build_targets: ["stage1", "stage2"]

An example pipeline running this can be found here:

https://gitlab.developers.cam.ac.uk/uis/devops/experiments/st981-test/lab-allocator/-/pipelines/609579