Add Phoenix entry to collection index

[Lwrless]

What type of PR is this?

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

Description

Add Phoenix devcontainer template to the index.

Collection checklist

• Collection name
• Maintainer name
• Maintainer contact link
• Repository URL
• OCI Reference
• I acknowledge that this collection provides new functionality, distinct from the existing collections part of this index.

[Kaniska244]

Looks fine. To be further reviewed by maintainers.