Switch to docker build

[aricodes-oss]

This uses a premade image to save you time on building releases and to make locally building a little easier. Currently I've got a job on one of my build servers that builds a fresh version of the toolchain weekly (https://github.com/aricodes-oss/kobo-toolchain), although it looks like the source repository hasn't been updated in quite some time so I may bump that to monthly.

This is one of my favorite pieces of software and you've done an incredible job with the user experience so I wanted to contribute in some small way. Thank you for helping reignite my love of reading!!

Closes #33

[NiLuJe]

A TC update is planned this month, but would be irrelevant for Nickel* stuff, as the nickel TC needs to be pinned to the matching, hilariously old GCC version.

Which itself needs to be pinned to an older binutils version, otherwise linking shenanigans ensue (broken as-needed behavior, which leads to overlinking, which is... bad enough, but even more so with Qt).

And since the glibc version also needs to be pinned for all of 'em, that leaves zero breathing room in terms of potential updates (until Kobo updates their stuff, anyway).

[shermp]

Hi, thank you for contributing @aricodes-oss

My preference is not to rely on non-official external docker images. I would prefer to create and host my own, or use an image provided by koxtoolchain (if they had a current one...). I'll have a think about how I want to proceed.

Note, I started a new job this week (and moved towns), so I don't have a lot of bandwidth available right now. I also don't currently have access to my dev PC.

[aricodes-oss]

My preference is not to rely on non-official external docker images. I would prefer to create and host my own, or use an image provided by koxtoolchain

Definitely understandable! The repository I linked contains a Dockerfile for making that base image and it can be uploaded to Github's container registry or the regular docker registry. That's just the only place I had upload access to at the time of making this PR.

And since the glibc version also needs to be pinned for all of 'em, that leaves zero breathing room in terms of potential updates (until Kobo updates their stuff, anyway).

Duly noted! I'll probably go axe the cron job on my CI system then since it seems like there's no need for me to maintain the image.