Xdebug always enabled if XDEBUG_ENABLE has empty value [has PR]

[AlexSkrypnyk]

Describe the bug
Currently, the developer experience to enable Xdebug is to uncomment
the line XDEBUG_ENABLE: true in docker-compose.yml..

This is annoying and not possible to set this variable on host (and automate it) like so in docker-compose.yml:

XDEBUG_ENABLE: "${XDEBUG_ENABLE:-}"

This line
https://github.com/amazeeio/lagoon/blob/master/images/php/fpm/entrypoints/60-php-xdebug.sh#L29
uses if [ ${XDEBUG_ENABLE+x} ];, which evaluates to boolean true when XDEBUG_ENABLE env variable is set to empty.

Proposed solution
Change the condition to check if XDEBUG_ENABLE variable value is empty instead of checking if the value is set.

[AlexSkrypnyk]

With PR merged now, it is possible to enable Xdebug like so XDEBUG_ENABLE=1 docker-compose build, provided that docker-compose.yml has this line:

x-environment:
  &default-environment
    XDEBUG_ENABLE: ${XDEBUG_ENABLE:-}

[AlexSkrypnyk]

If using Ahoy, the workflow to enable/disable XDebug is now greatly simplified:
ahoy debug to enable; ahoy up to disable.

in .ahoy.yml:

commands:
  up:
    cmd: docker-compose up -d "$@"
  debug:
    cmd: XDEBUG_ENABLE=1 ahoy up