added multistage docker builds with docker instructions in readme

[heywoodlh]

Round two of #618 😄

Changes:

• Multistage builds should reduce the size of the image (and decrease vulnerabilities)
• Removed hardcoding the version of nfdump, builds nfdump from source
• 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)
• Running as non-root user by default

Related #674

[heywoodlh]

If this gets merged, the following future additions would be nice:

• Images pushed to ghcr.io or Docker Hub
• GitHub Actions to regularly build, test and push images

[phaag]

The docker file is a 3rd party file. I'd happily add your PR but I do not check or verify the code.