Badge Request: GitHub Container Registry

:clipboard: **Description**

GitHub recently introduced the GitHub Container Registry (see: https://github.blog/2020-09-01-introducing-github-container-registry/). It would be nice if there were badges available similar to those that shields has for Docker image version, image size, and pulls, but for the new GitHub Container Registry.

:link: **Data**

I'm not sure if GitHub has an API for this yet.

:microphone: **Motivation**

Motivation is the same as that of the equivalent badges that already exist for Docker Hub, but targeting the GitHub Container Registry instead. Providing a nice concise visual summary of some of the key statistics one might be interested in with a Docker image, but for images available in the GCR.

No repository file, test, or entry point is named. First investigate whether GitHub Container Registry provides accessible data for image version, size, and pull counts, then locate the existing Docker Hub badge implementations for comparison. Done means agreeing on supported data and adding equivalent working badges with documented behavior and tests.

Written by the indexing model from the issue text.