add Couper devcontainer feature to the index

[malud]

What type of PR is this?

• [x] Add a new dev container collection
• [ ] Update to an existing dev container collection
• [ ] Documentation/spec update
• [ ] Other containers.dev site update (UX, layout, etc)

Related Issues

• Related Issue #
• Closes #

Description

This PR adds the Couper feature to the public index. This allows users to add the couper binary (via gh release) to their stack.

Collection checklist

If your PR contributes a new collection, please utilize this checklist:

• [x] Collection name
• [x] Maintainer name
• [x] Maintainer contact link (i.e. link to a GitHub repo, email)
• [x] Repository URL
• [x] OCI Reference
• [x] I acknowledge that this collection provides new functionality, distinct from the existing collections part of this index.