Add CLAUDE.md for AI-assisted development

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is the AppPack Codebuild Image - a minimal Docker image used by AWS CodeBuild for AppPack application builds. The project contains both the Docker image configuration and a Go-based build orchestration tool.
+
+## Key Commands
+
+### Docker Build
+```bash
+# Build the Docker image (linux/amd64 platform)
+make build
+
+# Build and tag with git version
+make build-and-tag
+
+# Push to ECR
+make ecr-login  # Login to ECR first
+make push       # Push with :builder tag
+make push-tag   # Push with git version tag
+```
+
+### Go Development (builder/)
+```bash
+# Run tests for the builder
+cd builder && go test ./...
+
+# Run specific test package
+cd builder && go test ./build/...
+
+# Build the Go binary
+cd builder && go build -o apppack-builder
+
+# Run with verbose/debug output
+export APPPACK_DEBUG=1
+```
+
+## Architecture
+
+### Core Components
+
+The project consists of two main parts:
+
+1. **Docker Image** - Minimal build environment for AWS CodeBuild
+   - Defined in `Dockerfile`
+   - Pushed to ECR: `public.ecr.aws/d9q4v8a4/apppack-build`
+   - Optimized for fast provisioning (< 25s)
+
+2. **Builder CLI** (`builder/`) - Go application that orchestrates builds
+   - Entry point: `builder/main.go` → `cmd/root.go`
+   - Commands: `prebuild`, `build`, `postbuild`
+   - Uses Cobra for CLI framework
+
+### Build System Architecture
+
+The builder supports two build systems:
+- **Docker builds** - Traditional Dockerfile-based builds
+- **Buildpack builds** - Using Cloud Native Buildpacks (Heroku/Paketo)
+
+Key configuration files parsed by the builder:
+- `apppack.toml` - Primary AppPack configuration
+  - Can be read from a custom location via APPPACK_TOML env var
+  - If read from custom location, it's automatically copied to `apppack.toml` after build for artifact archival
+  - The copy operation is handled by `filesystem.CopyAppPackTomlToDefault()` with proper error handling
+- `app.json` - Heroku-compatible app configuration
+- `metadata.toml` - Build metadata from buildpacks
+
+### AWS Integration
+
+The builder interacts with AWS services:
+- **SSM Parameter Store** - Fetches environment variables from configured paths
+- **S3** - Stores build artifacts and cache
+- **ECR** - Pushes built container images
+- **CloudFormation** - Updates stack parameters
+
+### Package Structure
+
+- `cmd/` - CLI command definitions
+- `build/` - Core build logic and configuration parsing
+  - `apppacktoml.go`, `appjson.go`, `metadatatoml.go` - Config file parsers
+  - `prebuild.go`, `build.go`, `postbuild.go` - Build phase implementations
+- `aws/` - AWS service integrations
+- `containers/` - Docker and buildpack container operations
+- `filesystem/` - Git and file operations
+- `shlex/` - Shell command parsing utilities
+
+## Build Process Flow
+
+1. **Prebuild** - Prepares environment, clones repository
+2. **Build** - Runs Docker or buildpack build based on configuration
+3. **Postbuild** - Pushes image to ECR, runs release tasks, updates CloudFormation
+
+## Default BuildSpec
+
+The following buildspec.yml is used by default in AWS CodeBuild:
+
+```yaml
+artifacts:
+  files:
+  - build.log
+  - test.log
+  - apppack.toml
+  - commit.txt
+  name: $CODEBUILD_BUILD_NUMBER
+phases:
+  build:
+    commands: apppack-builder build
+  install:
+    commands:
+    - if [ -z "$CODEBUILD_START_TIME" ]; then exit 0; fi
+    - echo "Starting Docker daemon..."
+    - nohup /usr/local/bin/dockerd --host=unix:///var/run/docker.sock --host=tcp://127.0.0.1:2375
+      --storage-driver=overlay2 --registry-mirror=https://registry.apppackcdn.net
+      &> /var/lib/docker.log &
+    - for i in $(seq 1 10); do docker info > /dev/null 2>&1 && break || sleep 0.5;
+      done
+  post_build:
+    commands: apppack-builder postbuild
+  pre_build:
+    commands: apppack-builder prebuild
+version: 0.2
+```
+
+Key points:
+- Docker daemon is started during install phase with AppPack's registry mirror
+- The three main phases map to `apppack-builder` subcommands
+- Artifacts include build/test logs, apppack.toml, and commit information
+
+## Environment Variables
+
+- `APPPACK_DEBUG` - Enable debug logging
+- `APPPACK_TOML` - Path to apppack.toml configuration file
+- `ALLOW_EOL_SHIMMED_BUILDER` - Allow end-of-life shimmed builders (for testing)
+
+## Development Guidelines
+
+- **Code Formatting**: Always run `gofumpt` before committing Go code changes
+- **Testing**: Verify all tests pass with `go test ./...` after making changes