docs: Document env var lookup for --build-arg

[devfbe]

Add more documentation for the --build-arg parameter of the executor. Describe that it's possible to lookup build argument values from the environment and that this might be useful if you don't want to fiddle around with shell escaping.

Description

Just improved the README section of the --build-arg parameter. I exactly needed the feature that you can lookup build arg values from the environment by just passing the build arg key. I only figured out that this is already possible because I red the source code - so I thought it's worth mentioning it in the documentation.

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Release Notes

Improved --build-arg parameter documentation

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[devfbe]

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