Draft: ci: #3627452 Allow running tests on FreeUnit instead of Apache

Select the PHP image's web server with _TARGET_SERVER ("apache" by default, or "unit") and its tag with _TARGET_IMAGE_TAG. When the image ships unitd, server-setup.sh starts FreeUnit with .gitlab-ci/unit.json instead of Apache.

unit.json mirrors the root .htaccess (denied files, the PHP entry point allow-list, clean URLs, pre-compressed CSS/JS aggregates, svgz) and is generated by .gitlab-ci/scripts/unit-config.py. To match mod_php it disables fastcgi_finish_request() (mod_php finishes the response only after kernel.terminate), sets assert.active=0 as .htaccess does, and accepts chunked request bodies.

By: andypost Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com

Closes #3627452