Add development environment setup with Docker and VS Code Dev Containers

contrib/docker/devcontainer/README.md

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+# Development Environment Setup
+
+This document describes how to set up a development environment for Yggdrasil using Docker and VS Code Dev Containers.
+
+## Prerequisites
+
+- Docker installed and running
+- VS Code with the "Dev Containers" extension installed
+- Git configured with your user information
+
+## Option 1: VS Code Dev Containers (Recommended)
+
+1. Open this project in VS Code
+2. When prompted, click "Reopen in Container" or:
+   - Press `Ctrl+Shift+P` (or `Cmd+Shift+P` on macOS)
+   - Type "Dev Containers: Reopen in Container"
+   - Select the option
+
+VS Code will automatically build the development container and set up the environment with:
+- Go 1.23 with all necessary tools
+- Language server (gopls)
+- Linting (golangci-lint)
+- Debugging support (delve)
+- Git integration
+- Zsh shell with Oh My Zsh
+
+## Option 2: Manual Docker Container
+
+If you prefer to use Docker directly:
+
+### Using Makefile commands:
+
+```bash
+# Build and run the development container
+make -f Makefile dev
+
+# Or build and run separately
+make -f Makefile dev-build
+make -f Makefile dev-run
+
+# Get shell access to running container
+make -f Makefile dev-shell
+
+# Stop the container
+make -f Makefile dev-stop
+
+# Clean up
+make -f Makefile dev-clean
+```
+
+### Using Docker directly:
+
+```bash
+# Build the development image
+docker build -f Dockerfile -t yggdrasil-dev .
+
+# Run the container
+docker run -it --rm \
+  --name yggdrasil-dev \
+  -v $(pwd):/workspace \
+  -v ~/.gitconfig:/home/vscode/.gitconfig:ro \
+  -p 9000:9000 \
+  -p 9001:9001 \
+  -p 9002:9002 \
+  -p 9003:9003 \
+  --privileged \
+  --cap-add=NET_ADMIN \
+  --device=/dev/net/tun \
+  yggdrasil-dev
+```
+
+## Development Features
+
+The development environment includes:
+
+- **Go Tools**: gopls, delve debugger, goimports, golangci-lint, staticcheck
+- **Editor Support**: Syntax highlighting, auto-completion, debugging
+- **Testing**: Go test runner and coverage tools
+- **Networking**: Privileged access for network interface testing
+- **Port Forwarding**: Ports 9000-9003 exposed for Yggdrasil services
+
+## Building and Testing
+
+Inside the container:
+
+```bash
+# Build the project
+./build
+
+# Run tests
+go test ./...
+
+# Generate configuration
+./yggdrasil -genconf > yggdrasil.conf
+
+# Run with configuration
+./yggdrasil -useconf yggdrasil.conf
+```
+
+## Tips
+
+1. Your local Git configuration is mounted into the container
+2. The workspace directory (`/workspace`) is mapped to your local project directory
+3. All changes made to source files are persistent on your host machine
+4. The container runs as a non-root user (`vscode`) for security
+5. Network capabilities are enabled for testing network-related features
+
+## Troubleshooting
+
+- If the container fails to start, ensure Docker has enough resources allocated
+- For network-related issues, verify that the container has the necessary privileges
+- If Go tools are missing, rebuild the container: `make -f Makefile dev-clean dev-build`