Add Docker pipeline

[IlyaVassyutovich]

Add proper¹ pipeline to build and push a lightweight (based on Alpine) Docker image with imapfilter; I expect it to replase this drastically outdated image at DockerHub.

¹ — free and open, with easily verifiable source code.

[lefcha]

I would like to avoid maintaining a docker setup inside the codebase. Instead such contributions can go in the Wiki.

And actually there's already a simple Docker setup page there: https://github.com/lefcha/imapfilter/wiki/Docker

Feel free to modify this page or add a new one with a more advanced/complicated setup if you like.

Also note that there's another similar open PR: https://github.com/lefcha/imapfilter/pull/300

[IlyaVassyutovich]

@lefcha Hello!

I've reworked Wiki-page for Docker support as you suggested, you can view it here. I'm open to suggestions and feel free to "pull" my version.

Unfortunately, GitHub does not support the same fork-pull-merge workflow for Wikis, so you can just copy-paste my version.

I've also integrated @mh166 suggestions.

I will commit current version from my Wiki-page to this branch now for convenience.

[IlyaVassyutovich]

Or, we can create a separate repo just for container image pipeline like other projects do. And push to docker hub, GHCR or whatever from there.