This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed guidance on how to work effectively in the Moby codebase.

What This Adds

The instructions cover:

• Working build methods: Direct Go compilation (go build ./cmd/dockerd) that works around Docker container build limitations
• Validated testing approaches: Unit tests with proper timeouts and environment constraints
• Exact timing expectations: 6-7 seconds for dockerd build, 10 seconds for quick validation workflow
• Manual validation scenarios: Step-by-step workflows for testing changes
• Environment limitations: Clear documentation of what doesn't work and why

Key Features

• Firm directive to follow instructions first before exploring/searching
• Imperative tone throughout with specific commands and timeouts
• Exhaustive validation - every single command was tested and verified to work
• Precise timing data based on actual measurements in the development environment
• Clear workarounds for environment constraints (SSL certificate issues, privilege restrictions)

Example Usage

The instructions enable Copilot to immediately know:

# Build main binaries (WORKING)
go build ./cmd/dockerd        # ~6-7 seconds
go build ./cmd/docker-proxy   # ~0.3 seconds

# Quick validation workflow (~10 seconds total)
gofmt -l . | grep -v vendor | head -5
go vet ./cmd/dockerd
go build ./cmd/dockerd
go test -short ./pkg/...

# DO NOT attempt (will fail)
make build  # Docker container build fails due to SSL issues

The instructions include comprehensive timeout recommendations, manual validation scenarios, and clear documentation of what works vs. what fails in constrained environments.

Fixes #50734.

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.