NIFI-8725 Added Single User Environment Variables for Docker

[exceptionfactory]

Description of PR

NIFI-8725 Adds support for specifying Single User authentication credentials using environment variables in NiFi Docker images. Additional updates include an example command and links to the Administration Guide section in the Docker README.

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[markap14]

Thanks @exceptionfactory was able to verify changes work exactly as expected. Makes it a lot easier to run this in docker! +1 merged to main.