updating dockerfile and readme with info on COPY and wildcards in Docker

[JamesEarle]

The old notation for the Docker COPY statement in the Dockerfile does not maintain directory structure when moved to the destination folder. This is fine for the aci-helloworld sample, but any other application with files in one or more directory will break as a result of this. I've updated the sample Dockerfile and provided a bit of detail into the COPY command and wildcard characters.