Add Docker quickstart guide

Concise guide covering:
- Build commands
- Basic and production run examples
- Environment variables table
- Custom SSL certs (own, mkcert, Let's Encrypt)
- Volumes and troubleshooting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/DOCKER_QUICKSTART.md

@@ -0,0 +1,157 @@
+# Docker Quickstart
+
+Get Stegasoo running in Docker in under 5 minutes.
+
+## Build
+
+```bash
+# Build web UI image
+sudo docker build -t stegasoo-web --target web .
+
+# Or build all targets
+sudo docker build -t stegasoo-api --target api .
+sudo docker build -t stegasoo-cli --target cli .
+```
+
+## Run (Basic)
+
+```bash
+# HTTP only, no auth
+sudo docker run -d \
+  -p 5000:5000 \
+  -e STEGASOO_AUTH_ENABLED=false \
+  --name stegasoo \
+  stegasoo-web
+```
+
+Visit http://localhost:5000
+
+## Run (Production)
+
+```bash
+# HTTPS + Auth + Channel Key
+sudo docker run -d \
+  -p 5000:5000 \
+  -e STEGASOO_AUTH_ENABLED=true \
+  -e STEGASOO_HTTPS_ENABLED=true \
+  -e STEGASOO_HOSTNAME=stegasoo.local \
+  -e STEGASOO_CHANNEL_KEY=ABCD-1234-EFGH-5678-IJKL-9012-MNOP-3456 \
+  -v stegasoo-data:/opt/stegasoo/frontends/web/instance \
+  -v stegasoo-certs:/opt/stegasoo/frontends/web/certs \
+  --name stegasoo \
+  stegasoo-web
+```
+
+Visit https://localhost:5000 (accept self-signed cert warning)
+
+## Environment Variables
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `STEGASOO_AUTH_ENABLED` | `true` | Require login |
+| `STEGASOO_HTTPS_ENABLED` | `false` | Enable HTTPS |
+| `STEGASOO_HOSTNAME` | `localhost` | Hostname for SSL cert |
+| `STEGASOO_CHANNEL_KEY` | *(none)* | Shared channel key (32 alphanumeric chars with dashes) |
+
+## Docker Compose
+
+Create `.env` file:
+```bash
+STEGASOO_AUTH_ENABLED=true
+STEGASOO_HTTPS_ENABLED=true
+STEGASOO_HOSTNAME=stegasoo.local
+STEGASOO_CHANNEL_KEY=
+```
+
+Run:
+```bash
+sudo docker-compose up -d web
+```
+
+## Custom SSL Certificates
+
+### Use Your Own Certs
+
+```bash
+# Stop container
+sudo docker stop stegasoo
+
+# Copy certs to volume
+sudo docker run --rm -v stegasoo-certs:/certs -v $(pwd):/src alpine \
+  sh -c "cp /src/your-cert.crt /certs/server.crt && cp /src/your-key.key /certs/server.key && chmod 600 /certs/server.key"
+
+# Start container
+sudo docker start stegasoo
+```
+
+### Use mkcert (Local Development)
+
+```bash
+# Install mkcert
+brew install mkcert  # macOS
+# or: sudo apt install mkcert  # Debian/Ubuntu
+
+# Create local CA and certs
+mkcert -install
+mkcert -cert-file server.crt -key-file server.key localhost 127.0.0.1 stegasoo.local
+
+# Copy to Docker volume (see above)
+```
+
+### Use Let's Encrypt (Public Server)
+
+```bash
+# Get cert
+sudo certbot certonly --standalone -d yourdomain.com
+
+# Copy to Docker volume
+sudo docker run --rm -v stegasoo-certs:/certs alpine \
+  sh -c "cp /etc/letsencrypt/live/yourdomain.com/fullchain.pem /certs/server.crt && \
+         cp /etc/letsencrypt/live/yourdomain.com/privkey.pem /certs/server.key && \
+         chmod 600 /certs/server.key"
+```
+
+## Volumes
+
+| Volume | Purpose |
+|--------|---------|
+| `stegasoo-data` | User database, settings |
+| `stegasoo-certs` | SSL certificates |
+
+## Smoke Test
+
+```bash
+# Check container logs
+sudo docker logs stegasoo
+
+# Test HTTP endpoint
+curl -k https://localhost:5000/health
+
+# Expected: {"status":"ok","version":"4.1.7",...}
+```
+
+## Troubleshooting
+
+**Container won't start:**
+```bash
+sudo docker logs stegasoo
+```
+
+**Out of memory:**
+```bash
+# Argon2 needs 256MB+ per operation
+sudo docker run --memory=768m ...
+```
+
+**Certificate errors:**
+```bash
+# Regenerate self-signed cert
+sudo docker exec stegasoo rm -rf /opt/stegasoo/frontends/web/certs/*
+sudo docker restart stegasoo
+```
+
+**Reset everything:**
+```bash
+sudo docker stop stegasoo && sudo docker rm stegasoo
+sudo docker volume rm stegasoo-data stegasoo-certs
+```