Support GitLab CI's 'image' dictionary format

[xanarin]

GitLab CI supports both a bare string and a dictionary for the image key. This is their reference for the possible values of an image key. Scuba should support this format (as much as it is able) so that a developer can !from_yaml .gitlab-ci.yml myjob.image without concern of Scuba not being able to understand the dictionary and its keys.

Some examples of legitimate uses of image according to GitLab's documentation:

• Bare string

  build_all:
    image: "xanarin/base-build:1.17"
    script: "make -j24"

• Dictionary with image name:

  build_all:
    image:
      name: "xanarin/base-build:1.17"
    script: "make -j24"

• Dictionary with entrypoint:

  build_all:
    image:
      name: "xanarin/base-build:1.17"
      entrypoint: ["/usr/bin/bash", "/opt/run_with_env.sh"]
    script: "make -j24"

• Dictionary with "pull policy" (can have value always, if-not-present, or never)

  build_all:
    image:
      name: "xanarin/base-build:1.17"
      pull_policy: always
    script: "make -j24"

• Dictionary with pull policy array

  build_all:
    image:
      name: "xanarin/base-build:1.17"
      pull_policy: [always, if-not-present]
    script: "make -j24"

I don't know if Scuba should to fully support the pull_policy directive (in the sense that it obeys the policy), but at a minimum I think Scuba shouldn't error out if it encounters that directive.