fix(build): old base image

[jokesterfr]

Questions	Answers
Description?	Fix the build of images with old distros for PHP 5.6 and add more tests in the CI workflow.
Type?	bug fix + enhancements
BC breaks?	yes. CUSTOM_BASE_IMAGE has been removed, as this behavior is now configurable for both base and final images through TARGET_IMAGE or TARGET_IMAGE_NAME + TARGET_IMAGE_TAG (new)
Deprecations?	no, but as the build script API change, please do not use CUSTOM_BASE_IMAGE nor --custom-base-image starting from now
Fixes	PrestaShop 8.3 now proposes a parameters.yml.dist file. TARGET_IMAGE related fixes confusing with base image variables.
How to test?	BASE_ONLY=true TARGET_PLATFORM=linux/arm64 PHP_VERSION=5.6 ./build.sh

[jokesterfr]

Should be good to go, will have a look at this next Tuesday.
I'm afraid some issues might raise with the new CI enforcements, will see :)