"name.Dockerfile.dockerignore" no longer applies to dev container build

• VSCode Version: 1.95.1
• Local OS Version: Pop!_OS 22.04
• Remote OS Version: n/a
• Remote Extension/Connection Type: Containers

When pointing VSCode at a named devcontainer Dockerfile, e.g dc.Dockerfile, then according to the Docker Spec, dc.Dockerfile.dockerignore should be used if it is found next to the named devcontainer dockerfile.

This used to work when I set up some dev containers around August/September. Now, it no longer does.

Repro is at https://github.com/jwhitaker-gridcog/devcontainers-repro , or briefly:

{
	"name": "Default Linux Universal",
	"build": {
		"dockerfile": "dc.Dockerfile",
		"context": ".."
	}
}

FROM mcr.microsoft.com/devcontainers/universal:2-linux

COPY . .

# testfile should be EXCLUDED from the docker context, as per dockerignore
RUN if [ -f testfile ]; then echo 'test failed'; exit 1; else echo 'all good'; fi

testfile

# blank file

Expected behaviour: the image in the repro should build successfully, as testfile should be excluded from its build context.