Pre-built image not used as-is

Apologies if this is the wrong place to ask, but would the proposal above address the fact that running devcontainer build followed by devcontainer open opens VS Code but then in the output one of the first things it does is start building stuff again?

[1690 ms] Start: Run: docker events --format {{json .}} --filter event=start
[1692 ms] Start: Run: docker-compose --project-name myproject -f /Users/username/Projects/myproject/docker-compose.yml build

It makes for a very slow startup experience since I cannot interact with the editor until the build completes. What is also non-intuitive for me is that even though it rebuilds after opening the dev container, I am sometimes still prompted that e.g. docker-compose.yml has changed and may need to be rebuilt. How many times does it need to build before VS Code is content that everything is in working shape and can let me get to work?

Originally posted by @jsma in https://github.com/microsoft/dev-container-spec/issues/21#issuecomment-1086338189