Building images using Azure Container Registry fails

When I start the image, login and try to build an image using az acr build fails:

docker run -ti --rm microsoft/azure-cli bash
bash-4.4# az login
To sign in, use a web browser to open the page https://microsoft.com/devicelogin and enter the code xxx to authenticate.
[
  {
    "cloudName": "AzureCloud",
    "id": "xxx",
    "isDefault": true,
    "name": "xxx",
    "state": "Enabled",
    "tenantId": "xxx",
    "user": {
      "name": "xxx",
      "type": "user"
    }
  }
]
bash-4.4# echo 'FROM nginx:mainline-alpine' > Dockerfile
bash-4.4# az acr build -t my-nginx --verbose -r xxxregistry .
'--file or -f' is not provided. './Dockerfile' is used.
Packing source code into tar to upload...
unexpected end of data

Building the image without using the Dockerimage works fine.