Refresh registry docs

[jkodroff]

In the pulumi/pulumi version of the workflow, we sync what became this repo's README with the Overview on each of our Docker Hub repos, e.g. https://hub.docker.com/r/pulumi/pulumi-base. These docs need a refresh:

• Each repo should have its own description and should start with a consistent Pulumi tagline, e.g. "Developer-First Infrastructure", and include a description of the Pulumi tool, and also a quick blurb on what the particular image is (e.g. Pulumi with Golang SDK installed). These should probably go in, e.g. docker/dotnet/README.md.
• Include information on where to submit issues.
• Include what platforms we publish and which tags we use.
• Re-add the task to sync the README for each image, ensuring that the content is published to both Docker Hub and ECR.