Building images using Azure Container Registry fails

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

```sh
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.