Add "docker.io/" registry prefix to docker compose image

[Valloric]

Required by Podman Compose. Docker Compose should not care either way. The registry prefix is explicitly supported in the compose file spec

[joel-jeremy]

@MatissJanis do you think we should point the docker compose file to the GitHub container registry or should we use docker hub instead?

[joel-jeremy]

@Valloric actual server images are also hosted on docker hub. Can we point the compose file there instead? Also, this needs to have a release notes to be merged. Please see https://github.com/actualbudget/docs#writing-good-release-notes on how to add one. :)

[Valloric]

I changed the prefix to point to docker hub and added the release notes. Should be good to go now.

Some friendly advice from someone who spent 15+ years maintaining various OSS projects: the release notes process adds needless overhead and will cut the rate of PRs you get by a factor of 10. This process is largely automatable at the time of cutting a release. The non-automatable parts (categorization of bugfix/feature/whatever) is not worth the overhead.

Do with that advice as you wish. :)

[joel-jeremy]

Thank you! We'll keep that advice in mind. :)