Refactor Docker image workflow with docker actions

[ranisalt]

Pull Request Prelude

• [ ] I have followed proper The Forgotten Server code styling.
• [ ] I have read and understood the contribution guidelines before making this PR.
• [ ] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Uses docker actions to setup, login, build and push docker images to the GitHub container registry. Images will be pushed to the latest tag on every master commit, and to tags matching git tags when a new git tag is pushed.