This rule is activated by specifying a label-schema, where one label must contain a URL:

label-schema:
  website: url

or via command line:

$ hadolint --require-label website:url Dockerfile

LABEL website="not a url"

LABEL website="https://www.example.com"

A label-schema may require a label to contain some URL, for example to ensure compliance to standardized specifications.

This rule does not expand variables. It requires the value of the label to be a literal URL.