Update calibre image to include ebook-convert#29

Calibre web will be dropping support for kindlegen soon, so it wouldn’t be possible to convert books. There is a docker image with the calibre converter installed which we can use - https://hub.docker.com/r/technosoft2000/calibre-web/ . The current image that we have is based on alpine and it can’t run the precompiled calibre binaries even if they are coppied to it (which works for kindlegen).