Add Flutter

[ThePhaseless]

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)

Related Issues

• Related Issue 426#

Description

So far, I've created somewhat self-sufficient Dockerfile for developing flutter apps for Android, Web and Linux platforms. Previous feature and devcontainer are not mantained anymore. I will prob. add more devcontainers if needed.

Collection checklist

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

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

[samruddhikhandale]

Thanks! ⚡