feat: multi-arch Docker images built with Nix

[tembleking]

Summary

• Replace Dockerfile and .dockerignore with pure Nix image builds (dockerTools.buildLayeredImage + pkgsCross)
• Build linux/amd64 and linux/arm64 images via nix build
• Publish workflow pushes images by digest (via skopeo) and assembles multi-arch manifests with docker buildx imagetools create
• Test workflow runs both arch builds in a matrix; functional test and scan run on amd64
• Add skopeo to dev shell and use nix develop --command bash {0} as default shell in CI jobs

Test plan

• Test Image Build workflow passes for both matrix entries (amd64 + arm64)
• Verify nix build .#sysdig-mcp-server-image-amd64 produces a loadable image locally
• Verify nix build .#sysdig-mcp-server-image-aarch64 completes successfully

[Copilot]

Pull request overview

This PR migrates Docker image builds from a traditional Dockerfile to a pure Nix-based approach using dockerTools.buildLayeredImage, enabling native multi-architecture support for both linux/amd64 and linux/arm64.

Changes:

• Replaced Dockerfile and .dockerignore with Nix-based image definitions (docker.nix, docker-base-amd64.nix, docker-base-aarch64.nix)
• Updated CI workflows to build and test both architectures using a matrix strategy, with digest-based pushing and multi-arch manifest creation
• Added development tooling (nix-prefetch-docker, skopeo) and a justfile recipe to update base images

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
justfile	Adds recipe to fetch and update base image metadata for both architectures
flake.nix	Defines multi-arch image packages using pkgsCross and adds required dev tools
docker.nix	Implements Nix-based Docker image builder with architecture-specific base image selection
docker-base-amd64.nix	Defines metadata for the amd64 base image
docker-base-aarch64.nix	Defines metadata for the aarch64 base image
Dockerfile	Removed traditional Dockerfile in favor of Nix approach
.dockerignore	Removed as it's no longer needed with Nix builds
.github/workflows/test_image.yaml	Updated to build and test both architectures in a matrix with Nix
.github/workflows/publish.yaml	Refactored to build arch-specific images, push by digest, and create multi-arch manifests

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 4 comments.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 10 changed files in this pull request and generated 6 comments.

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[Copilot]

Pull request overview

Copilot reviewed 13 out of 14 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 3 comments.

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[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 3 comments.

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[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated no new comments.

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