Updated to Alpine v3.12.0

New version v1.5.0

 * new base image [technosoft2000/alpine-base:3.12-1](https://hub.docker.com/r/technosoft2000/alpine-base/) based on Alpine 3.12.0
 * fixed `[WARNING]: Empty continuation line found in: ...` in Dockerfile
 * upgrade of [sgerrand/alpine-pkg-glibc](https://github.com/sgerrand/alpine-pkg-glibc) to version 2.32-r0
 * changed glibc compiled package (ArchLinux) `libutil-linux` to new `util-linux-libs` - see issue **Build failure in v1.4.1** #87
 * updated dependencies from `requirements.txt` and `optional-requirements.txt` - see issue **Missing jsonschema in the docker image** #90
 * changed python from `python2` to `python3`
 * allow **gevent 20.9.0** otherwise:
   - error will appear `<frozen importlib._bootstrap>:219: RuntimeWarning: greenlet.greenlet size changed, may indicate binary incompatibility. Expected 144 from C header, got 152 from PyObject`
   - see at `gevent` where the issue was known: https://github.com/gevent/gevent/issues/1260
 * added **unrar**
 * **kepubify** is not added, because Alpine Linux doesn't contains a pre-build package at the moment
 * updated Calibre **ebook-convert** from 4.13.0 to 5.3.0
 * uses `https://github.com/Technosoft2000/docker-calibre-web/releases/download/kindlegen/kindlegen_linux_2.6_i386_v2_9.tar.gz` to download Amazon kindlegen