Delete a Docker image from inside an RSpec script

Question

TL;DR: I can't delete my Docker image from inside my RSpec script, because RSpec still has a dependent container

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I'm using RSpec to test my Dockerfiles. That is, to test whether the Dockerfile results in a correct image. My test script does this by building an image from the Dockerfile, and then letting RSpec test whether a container (based on that image) has the correct contents.

The execution flow is as follows:

1. My RSpec script builds the Docker image (in its before(:all) block)
2. Serverspec creates a Docker container (based off the Docker image)
3. RSpec / Serverspec executes the tests inside the Docker container
4. RSpec / Serverspec calls my RSpec script's after(:all) block
5. RSpec / Serverspec deletes the Docker container

I tried to delete the image from inside the after(:all) block (or the after(:context) block, but that has the same issue), as that is the last hook that RSpec offers me. Only I can't, because the Docker container has not been deleted yet.

Force-deletion is no solution, as Docker then keeps an unnamed, untagged version of the image around.

Is there an (idiomatic) way to accomplish this?

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Example RSpec script:

require "serverspec"
require "docker-api"

describe "Dockerfile" do

  image_name = 'my_image'
  image_tag = 'my_tag'
  image_name_tag = image_name + ':' + image_tag

  before(:all) do
    set :os, family: :debian
    set :backend, :docker

    # Build image
    @image = Docker::Image.build_from_dir("../dockerfile/")
    @image.tag(repo: image_name, tag: image_tag)

    # Provide Serverspec with our image's id
    set :docker_image, @image.id
  end

  it "is a Debian-based container" do
    expect(os_version).to include("Debian")
  end

  def os_version
    command("cat /etc/*release").stdout
  end

  after(:context) do
     # Clean up
     @image.delete(name: image_name_tag) # <-- this fails
  end
end

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(It's not entirely clear to me where RSpec's responsibilities end and Serverspec's begin; hence the RSpec / Serverspec above.)