chore!: extract image-related code to the top-level image package

[mdelapenya]

• chore: extract dockerignore to the image package
• chore: extract image list to the image package
• chore: extract save images to the image package
• chore: extract image substitutors to the image package
• chore: extract image build to the image package

What does this PR do?

This PR refactors the code to extract the docker image related code to a top-level package: image.

This package will be responsible of:

• pulling images
• saving images as tar files
• parsing dockerignore files for image build
• listing images
• calculating image substitutors
• building images

The related tests have been moved to the package, but there are "e2e" tests (they are actually using a ContainerRequest) that have been kept in the root package of the library.

Why is it important?

Separation of concerns, reducing the API surface in the root package.

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