Add docker

[ntnn]

I've been maintaining docker-imapfilter for a couple years, which provides nightly and tagged builds of imapfilter as a docker image:

https://github.com/ntnn/docker-imapfilter
https://hub.docker.com/r/ntnn/imapfilter

I've added pushing to ghcr.io today (see https://github.com/ntnn/docker-imapfilter/pkgs/container/docker-imapfilter) and thought that since this requires no additional setup you might be interested in an official Docker Image.

Also I'd prefer that docker-imapfilter is a superset of an official Docker Image, rather than building imapfilter from the ground up.

I'd be open to maintain the Dockerfile and GitHub Action so it's not additional work. PRs against the Dockerfile and Action could be automatically assigned to be reviewed by me using a GitHub CODEOWNERS file:
https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

IIRC that does not require people in CODEOWNERS to be maintainers.

The file would basically be:

* @lefcha
Dockerfile @ntnn
.github/workflows/docker.yml @ntnn

For the Dockerfile and Action I abriged the ones used in docker-imapfilter:

https://github.com/ntnn/docker-imapfilter/blob/main/Dockerfile
https://github.com/ntnn/docker-imapfilter/blob/main/.github/workflows/image-build.yml

The Action builds also for ARM as this makes the image usable on single board computers like the Raspberry Pi (which is where I run docker-imapfilter on, e.g.).

[ntnn]

Note: I'd previously suggested including the Dockerfile in this request for docker: #243 (comment)

[ntnn]

https://github.com/ntnn/imapfilter/pkgs/container/imapfilter

That's roughly what it would look like in the ghcr for this repository. Note that no latest tag was pushed because of this conditional, which only allows latest to be set on tags:
https://github.com/lefcha/imapfilter/pull/300/files#diff-3f5366f6d6df3ec1179e5efadc6f350bfa88eebf4e2da589b4d94ccb85ae5e94R37

[rafo]

An "official" docker image would be great, but IMHO without the need to manage the imapfilter configuration in a git repo. I have parts of my config in a git repo, but the config used by imapfilter every 15min resides on my server. As an extra, why not. But I think its more save, robust and easy to use it without the need to connect to a git repo.

[ntnn]

An "official" docker image would be great, but IMHO without the need to manage the imapfilter configuration in a git repo. I have parts of my config in a git repo, but the config used by imapfilter every 15min resides on my server. As an extra, why not. But I think its more save, robust and easy to use it without the need to connect to a git repo.

Yeah, I'd keep docker-imapfilter separate - I'd just base it off of the official imapfilter image rather off of alpine (and skip building imapfilter :D).

[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: #295