feat(docker): build and push both amd64 and arm64 platforms

[hclaria]

Hey team 👋

First off : huge thanks for building ClickHouse and kubenetmon maintainers.
This great tooling. I’ve been using kubenetmon to measure cross-AZ traffic on my EKS clusters, and it’s promising to be incredibly useful ❤️

That said, I ran into a small issue while running it on ARM-based nodes (AWS Graviton).

The published images on GHCR were amd64-only, so exec format error popped up on ARM64 nodes.
Digging a bit deeper, it turned out that although PR #13 added multi-arch build support, the workflow still used load: true and push: false, which meant only the amd64 image was ever pushed to GHCR (the ARM64 build never made it into the manifest).

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🧩 The PR in a nutshell

• Builds both amd64 and arm64 images with Buildx
• Keeps the amd64 local load for Helm chart testing (via kind)
• Adds a proper multi-arch push in the docker-publish job
• Ensures GHCR now receives a manifest list with both platforms

I also kept caching between jobs via artifacts so we don’t rebuild everything twice : it’s still fast, and tests run unchanged.

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✅ Result

• kubenetmon now runs on ARM64 ( tested on Graviton without any hiccup)
• No more exec format error when deploying on mixed-architecture clusters
• CI/CD flow stays efficient because caching is preserved between jobs, and Helm tests still run on amd64
• Future releases (main and v* tags) will automatically publish proper multi-arch images to GHCR
• Users and downstream projects can now confidently deploy kubenetmon on any architecture without rebuilding images themselves

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Thanks again for maintaining kubenetmon and all the great work you’re doing in the ClickHouse ecosystem 🙌
Hopefully this PR saves a few headaches for folks running modern clusters on ARM ❤️

[CLAassistant]

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