Improve Codespace Load Times

[owenwahlgren]

• Move the Dockerfile to a hosted image instead of building it from within the Codespace.
• Migrate the "features" section for devcontainer.json directly into the Docker image (large load time improvement). Load times improved from average ~5 minutes -> ~2 minutes

https://ghcr.io/owenwahlgren/avalanche-starter-kit (I don't think moving the image to a host improves load time... the big improvement is from moving the "features" directly into the image. Maybe we should keep the Dockerfile in the repo to avoid confusion)

[owenwahlgren]

do not merge in middle of hackathon as it uses new etna cli