Change docker health check image to the new nanoserver one

Requested by Gaston in this review: https://reviews.apache.org/r/65127/ 

Right now, the docker health checks use `microsoft/powershell:nanoserver` to perform the health checks. The new windows insider container images have curl include, so we can use that when the new Windows update is out. Since we don't rely on powershell anymore, it should reduce the image size and improve performance.