Document images in icr.io

[leochr]

• Create a page similar to Liberty to list the images in icr.io and link it from the main readme
  https://github.com/OpenLiberty/ci.docker/blob/main/docs/icr-images.md
• Include icr.io as an option along with Docker Hub
• Update the example Dockerfiles to use images from icr.io

[scottkurz]

@leochr - Maybe #298 would be helpful to merge as a partial explanation in the meantime?

[leochr]

@scottkurz Yes, looks good. Thank you