added multistage docker builds with docker instructions in readme

[heywoodlh]

Hi

I made some improvements to the Docker build:

• Multistage builds should reduce the size of the image (and decrease vulnerabilities)
• Removed hardcoding the version of nfdump
• Separate targets for nfcapd and nfdump (i.e. one target for being the server and one for the client)
• Copy the repo source into the image instead of downloading and extracting an archive of nfdump
• Added documentation for building the image (Alpine being the assumed base build)

Please let me know if I can clarify anything with these changes. :) Thanks for the great project.

[phaag]

Thanks for the pull request.
Would you mind to create a readme.md file in the docker directory and copy the instructions there, reducing the primary readme with a reference to the docker readme?
Furthermore, don't hesitate to put your name and email as the contributor to the docker readme. :)