Add multi-arch Docker image builds and automated DockerHub releases

[ybhaw]

Hi

I noticed that the dockerhub image for this repo has not been updated in last 5 years and also there is no documentation around it in README.md. I have a few proposals around this:

1. Add a multi-stage Dockerfile:

• build stage: compiles massdns from source (similar to current Dockerfile)
• run stage: minimal image with only the binaries

2. Add a GitHub Actions workflow to:

• Build and push to DockerHub on every tagged release (e.g. v1.x.x)
• Use docker/buildx for multi-arch support: linux/amd64, linux/arm64, linux/arm/v7
• Push both versioned tags and latest

3. Add a Docker section to README.md covering:

• Pull instructions
• Example docker run usage

Why

• Enables users to run massdns without a local build environment
• Multi-arch covers ARM servers and Raspberry Pi setups
• Automated releases remove manual DockerHub update burden from maintainers

Additionally, the existing Dockerfile uses alpine:edge as the base image which could break things. I’d suggest switching to alpine:latest (or a pinned stable version like alpine:3.21) for more predictable builds.

Willing to contribute
I'll be happy to open a PR with these changes if you’re open to it.