Calibre - Docker mod for Ubuntu-based x86_64 containers

This mod adds the calibre binary to calibre-web, or other Ubuntu-based containers (x86_64 only), for ebook conversions.

In calibre-web docker arguments, set an environment variable DOCKER_MODS=linuxserver/mods:universal-calibre to enable.

If adding multiple mods, enter them in an array separated by |, such as DOCKER_MODS=linuxserver/mods:universal-calibre|linuxserver/mods:other-mod