Add GitHub workflow for multi-platform builds

[thaddeuskkr]

This PR is an updated version of #258 (my own previous PR). The self-hosted version of Lanyard currently only runs on amd64 servers. This PR adds multi-platform builds using GitHub-hosted runners, which allows Lanyard to be self-hosted on a wider variety of servers, especially as ARM increases in popularity. It specifically targets the following platforms, but more can be added in the future:

• linux/amd64
• linux/arm64

This PR should be able to be merged without any additional changes to the repository - it uses the existing Docker secrets that are already used in the previous version of the workflow.

I've tested the build process on my own branch, this can be found here. Additionally, I'm running the ARM64 container on my own instance of Lanyard (available here).

Merging this PR also resolves #244.

Docker Hub Images: https://hub.docker.com/r/thadkkr/lanyard

[Copilot]

Pull request overview

Adds a new GitHub Actions workflow intended to build and publish multi-platform Docker images (amd64 + arm64) for Lanyard, replacing the existing single-arch Docker workflows.

Changes:

• Introduces a docker-bake.hcl configuration with targets for local and multi-platform builds.
• Adds a new consolidated workflow (.github/workflows/docker.yml) that builds per-platform images and merges them into a multi-arch manifest.
• Removes the previous Docker build/push workflows (docker-image.yml, docker-build.yml).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
docker-bake.hcl	Adds bake targets for local and multi-platform builds.
.github/workflows/docker.yml	New CI workflow for building/pushing per-platform images and creating a manifest list.
.github/workflows/docker-image.yml	Removes prior push-on-main workflow.
.github/workflows/docker-build.yml	Removes prior PR build workflow.

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