Add a devcontainer template for Chromia's Rell language

[Thomas-Chromaway]

See https://docs.chromia.com for more on Rell.

This contains Java (from the base image), the Chromia tooling, a Postgres instance for storage, plus various useful tools. It is suitable for use with Github CodeSpaces and similar hosted environments, although as always valuable private keys should not be stored there.

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)

Description

Please replace this line with a description of your PR.

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.