build(docker): unify video clipper and php-fpm containers, switch to debian

docker/production/.gitlab-ci.yml

docker/production/app/Dockerfile

docker/production/app/entrypoint.sh

docker/production/app/supervisord.conf

docker/production/castopod/Dockerfile

docker/production/common/crontab.txt

docker/production/video-clipper/Dockerfile

docker/production/video-clipper/entrypoint.sh

docker/production/web-server/Dockerfile

docker/production/web-server/nginx.template.conf

docs/src/getting-started/docker.md