[CI] Use precise terminology for image components

[hcho3]

From #11079 (comment):

In general, I recommend following this type of naming for the variables involved here:

• IMAGE_REGISTRY = the place to pull the images from, like
  • (e.g. docker.io, quay.io, ghcr.io, an ECR URL)
• IMAGE_REPO = the name of the image, called a "repository" on some registries
  • (e.g. xgb-ci.jvm, dask/dask-notebook)
• IMAGE_TAG = ID that separates images from the same registry and repo
  • (e.g. 2024.10, main, stable)
• IMAGE_URI = fully-qualified identifier of the form ${IMAGE_REGISTRY}/${IMAGE_REPO}:${IMAGE_TAG}
  • (e.g. quay.io/pypa/manylinux2014_aarch64:latest)

cc @jameslamb

[jameslamb]

oh cool thanks for doing this!