Allow to customise reaper image, or use fully qualified image name

[cardil]

testcontainers-go/internal/config/config.go

Line 14 in c100f88

const ReaperDefaultImage = "testcontainers/ryuk:0.12.0"

I tried using testcontainers on a system with Podman, and I can't spawn it due to the line above. With Podman on many OSs, the testcontainers/ryuk:0.12.0 doesn't automatically resolve to docker.io/testcontainers/ryuk:0.12.0. I can't configure testcontainers to use docker.io/testcontainers/ryuk:0.12.0, because the value is an internal constant.

To solve this, I see two options:

1. Simply change the constant to docker.io/testcontainers/ryuk:0.12.0, which would work on all the systems, even on those that don't default to Docker Hub as the default registry.
2. Allow for customization of the reaper image.

[ianvernon]

Would appreciate this being prioritized. It seems quite simple to do, and blocks cases where we want to reap images in our CI infrastructure but need to mirror the ryuk image to an internal repository.