docker bake fails when compose test files exist without a dockerfile

https://github.com/balena-io/sshproxy/actions/runs/4876120556/jobs/8699234734#step:18:207

Currently if Flowzone sees a docker-compose.test.yml it will assume a bake is required and generate a default bake file, even if no Dockerfile exists.

Not sure what the best solution would be though...