Add pluggable image name substitution #1570

What happens

The only way to redirect image pulls is TESTCONTAINERS_HUB_IMAGE_NAME_PREFIX, which prefixes Docker Hub image names. It cannot:

• map other registries (ghcr.io, quay.io, mcr.microsoft.com) to a mirror
• apply different rules to different images
• rewrite a tag or a digest

Organisations that mirror everything through an internal registry end up editing image names in every test.

Expected

A function that every image name passes through before it is pulled, registered in configuration:

TestContainers.configure({
  imageNameSubstitutor: (image) => (image.registry === "ghcr.io" ? image.withRegistry("mirror.example.com/ghcr") : image),
});

The existing prefix variable becomes the default substitutor.

Where

• packages/testcontainers/src/container-runtime/image-name.ts
• the places that build ImageName for Ryuk and SSHD

Notes

• Java has a pluggable ImageNameSubstitutor, with PrefixingImageNameSubstitutor as the built-in equivalent of the prefix variable. Go has WithImageSubstitutors.
• Decide whether images named in Compose files and Dockerfile FROM lines are in scope.

Dependencies

• Blocked by Add programmatic configuration #1569: the substitutor is registered through configuration.