Migrate from searx to searxng because searx-docker is no more maintained (#28)

* Migrate from searx to searxng because searx-docker is no more maintained

* Add test config for SearXNG

* Fix test config

* Add SEARXNG_IMAGE_VERSION to .env.default

* Remove MORTY* vars from .env.default

* Apply suggestions from code review

* Fix test config

* Apply suggestions from code review

Co-authored-by: Tom Moulard <tom@moulard.org>

* Fix test config

Co-authored-by: Tom Moulard <tom@moulard.org>