anki: create a volume

[SandaruKasa]

Analogous to https://github.com/ankicommunity/anki-devops-services/blob/2218ed5f857c42085fd7091ea36f5b7c73f2ffae/services/anki-sync-server/images/Dockerfile#L35

Doing VOLUME here does not really prevent docker-compose from overriding it. This is similar to how EXPOSE does not really matter for docker-compose's ports. But at least there will always be a volume by default, lowering chances of misconfiguring the thing and losing the data together with the container.

The only doubt I have that SYNC_BASE here is ENV and not ARG. Maybe it would make more sense to switch it to ARG. I personally can't really see a use-case for being able to overwrite the sync path inside the container anyways. If one wants to do something different with it, they should probably just mount a volume on top of the hardcoded path.

[LuckyTurtleDev]

Maybe it would make more sense to switch it to ARG

I see not the benefits of using ARG over ENV. ARG would only be available at build time so, we need en ENV anyway.