Add support for automatically passing environment variable CI=true as a predefined ARG

[rstanevich]

Currently, to use CI as a build argument, it must be explicitly declared with an ARG directive in the Dockerfile:

ARG CI

Desired Behavior:

For example, allow CI=true to be passed automatically when specified via --opt=build-arg:CI=true without requiring the explicit ARG declaration, similar to how http_proxy, https_proxy, etc. are handled.

Rationale:

CI=true is a common convention across CI engines. Development tools and utilities may adjust their behavior when detecting they're running in a CI environment. And most Dockerfile's RUN commands execute those dev tools.

Having the ability to set CI=true available in the environment without requiring an explicit ARG declaration would streamline configurations and ensure/enforce consistent behavior

Refs:

• https://github.blog/changelog/2020-04-15-github-actions-sets-the-ci-environment-variable-to-true/
• https://answers.netlify.com/t/new-ci-true-build-configuration-treating-warnings-as-errors-because-process-env-ci-true/14434
• https://stackoverflow.com/questions/64289344/what-does-github-actions-environment-variable-ci-stands-for