Skaffold complains about the docker file being outside the docker build context

### Expected behavior
That skaffold has no more restrictions on the location of the dockerfile than docker build has
### Actual behavior
Skaffold complains that the dockerfile is outside the build context
### Information

```yaml
apiVersion: skaffold/v2beta4
kind: Config
metadata:
name: hobbes
build:
tagPolicy:
sha256: {}
artifacts:
- image: az
context: workers/az.worker/
docker:
dockerfile: ../../docker/Dockerfile.worker
```

### Steps to reproduce the behavior
any setup where the docker file is outside of the specified context. If the context folder doesn't exist Skaffold picks the right dockerfile and starts the build and will only fail if the the build tries to get files from the context