feat(docker): replace all-in-one image with FrankenPHP and Caddy based image + discard other images

• use serversideup/php as a base image
• remove nginx unit base
• remove app / webserver images
• add bundle stage to remove pipeline dependency
• update docker instruction docs

fixes #484 (closed), #545 (closed), #446 (closed), #398 (closed), closes #410 (closed), #358 (closed).