Add "function_exists" wrapper around "getenv_docker"

[tianon]

... to protect against WP-CLI loading it twice

Fixes #588

[tianon]

@GoldenMickey any way you could help us test this fix? 🙏

[GoldenMickey]

Sorry for the late response. How can I test it ? Does modifying the "FROM" clause of the dockerfile is enough ?

[tianon]

I guess if you've got a local install that's affected, you can download the wp-config-docker.php file from this PR and embed it in your volume as wp-config.php which should allow you to test whether WP-CLI still throws the warning/error, but if you're doing this through some automated process you can build an image directly from this PR with something like docker build --pull --tag 'wordpress:php7.4-fpm-alpine' 'https://github.com/docker-library/wordpress.git#pull/590/merge:latest/php7.4/fpm-alpine'

[GoldenMickey]

Fixed it for me after updating with wp-config-docker.php. Thank you!