Fix: Add missing dot in Docker build command

[WillScarlettOhara]

It wasn't immediately clear to me what was causing the issue. Adding the dot makes it more explicit for beginners. This gives them a simple, ready-to-use command.

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This change is 

[Frenzie]

Does . build all the Docker files in the directory? <custom_docker_file> is intended as a placeholder for the relevant Dockerfile.

[WillScarlettOhara]

The dot just makes the command work in the current directory. Without it, I had an error like in this example:

docker build -f Dockerfile -t vnckoappimage
ERROR: "docker buildx build" requires exactly 1 argument.
See 'docker buildx build --help'.

Usage:  docker buildx build [OPTIONS] PATH | URL | -

Start a build

[WillScarlettOhara]

I've added a few instructions and notes. Keep what you think is useful/relevant. 7e1ce7a

[Frenzie]

I'm not really sure what this means. Shouldn't you simply update the one above if something about it is outdated?

[WillScarlettOhara]

I thought there might be a reason to work with a 2020 version of KOReader, but maybe I'm wrong. It's not my repo so I'd rather add than replace. It's up to you. I'm just sharing my meager experience of what I ended up with today after following your instructions.
In any case, with the current code, if you only modify the KOReader version for a newer one, without changing the rest of the code, you end up with dependency problems and linux versions that are too old. That's why I used Debian Bullseye and installed these dependencies.

[Frenzie]

Ohh, now I get it, well, the problem anyway. The latest tag hasn't been updated in a while because it's not done automatically anymore. Maybe it shouldn't even be there at all. The correct current image is koreader/koappimage:0.4.4-20.04.

But all old versions still work on the latest systems.