Add CLAUDE.md with project context for AI-assisted development

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alderg

CLAUDE.md

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+# CLAUDE.md
+
+## Project overview
+
+Docker packaging for [draw.io](https://github.com/jgraph/drawio). Produces the `jgraph/drawio` and `jgraph/export-server` Docker images.
+
+## Repository structure
+
+- `main/` — Primary draw.io Docker image (Tomcat + draw.io WAR built from source)
+  - `Dockerfile` — Multi-stage build: builds draw.io WAR with Ant, deploys to Tomcat 9
+  - `docker-entrypoint.sh` — Runtime configuration via environment variables (PreConfig.js / PostConfig.js generation, SSL setup, Tomcat context path)
+- `image-export/` — Export server image (Node.js + Puppeteer + Chrome for PDF/image export)
+- `self-contained/` — docker-compose for running draw.io with export server and all integrations
+- `nextcloud/` — docker-compose for Nextcloud integration with nginx reverse proxy
+
+## Key conventions
+
+- The main branch is `dev`.
+- Commit messages reference upstream issues as `[jgraph/drawio#NNN]` and docker-drawio issues as `[jgraph/docker-drawio#NNN]`.
+- Environment variables are the primary configuration mechanism. They are processed in `main/docker-entrypoint.sh` which generates JS config files injected into the draw.io webapp at container startup.
+- The upstream draw.io source is cloned at build time (not vendored). The WAR is built from `https://github.com/jgraph/drawio`.
+
+## Building and testing
+
+```bash
+# Build the main image
+docker build -t jgraph/drawio main/
+
+# Build the export server image
+docker build -t jgraph/export-server image-export/
+
+# Run locally
+docker run -it --rm -p 8080:8080 jgraph/drawio
+
+# Run self-contained (with export server)
+cd self-contained && docker-compose up
+```
+
+## Environment variable patterns
+
+New environment variables should:
+1. Be documented in `README.md` under "Environment variables"
+2. Be processed in `main/docker-entrypoint.sh`
+3. Follow the `DRAWIO_*` naming convention for draw.io-specific settings
+4. Use shell defaults: `${VAR:-default_value}`