Environment variables can now be used to configure any
config property including the feeds list.
This makes it easier to use in environments without access
to config files like GCR. Some may prefer not to have a separate
config file as well.
Also added build metadata to the docker image and binaries.
Working *.epub to *.kepub.epub file conversions when
using a Kobo reader. Updated docker file to include
`kepubify` to convert to kepub. If not available the
file is just sent without conversion.
