You haven't answered the question, it doesn't take days if you use a standard dockerfile, and it can be used by team members who don't use vscode.
You can do exactly what you're saying--have a dockerfile as the source of truth--right now. And you can install the devcontainer CLI and use it outside vs code if necessary too: https://code.visualstudio.com/docs/devcontainers/devcontaine...
On GitHub codespaces it gives you a button on the website that is literally one click and get vs code in the browser connected to your dev container launched in their cloud--you don't even need vs code or anything installed and can code from a tablet for example.
However
> vs. spending days installing tools from a (likely) undocumented and out of date process
This makes no sense, that is not an advantage over a dockerfile.