modify dockerfile to switch to debian for whiteblock build

[n8wb]

Switched the dockerfile to use Ubuntu instead of Alpine for compatibility with the Whiteblock platform. The resulting image size is 259MB. If this is way to large, iputils-ping iperf3 can be removed from the installed deps to reduce the image size, but will render commands ping and iperf unusable. I can also put this in a file called whiteblock.dockerfile if you would like to keep the current Dockerfile.