How to specify docker build context that does NOT contain dockerfile (ContainerImage.fromAsset)?

[dev-samples]

Using vanilla docker I can build my image with:

$ pwd
~/projects/my-repo/infra/my-infra
$ docker build -f ../../docker/services/backend/Dockerfile ../..
...
 => => writing image sha256:bf...
0.0s

So the docker build context is the root of the repo:

my-repo/

while the dockerfile is in:

my-repo/docker/services/backend/Dockerfile

I am now trying to do the same using a cdk FargateTaskDefinition:

$ pwd
~/projects/my-repo/infra/my-infra
$ cdk deploy
**ERROR: could not find docker/services/backend: stat docker/services/backend: no such file or directory
fail: docker build --tag cdkasset-***bc4ab --file docker/services/backend/Dockerfile --

where:

~/projects/my-repo/infra/my-infra/lib/constructs/my-service.ts

contains:

        taskDefinition.addContainer('test', {
            image: ecs.ContainerImage.fromAsset("../../", {
                //file: "../../docker/services/backend/Dockerfile",
                file: "docker/services/backend/Dockerfile",
            }),
            //workingDirectory: "../",
        });

Am I doing something wrong or is it currently not supported to build an image where the docker build context does NOT contain the dockerfile:

#3342

?