docker-debugger

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# Docker Debugger

## Instructions

When debugging Docker issues:

1. **Identify the problem type**: Build, runtime, networking, or performance
2. **Gather information** using diagnostic commands
3. **Analyze logs and errors**
4. **Apply fixes**

## Diagnostic Commands

```bash
# Check running containers
docker ps -a

# View container logs
docker logs <container_id> --tail 100 -f

# Inspect container
docker inspect <container_id>

# Check resource usage
docker stats

# View container processes
docker top <container_id>

# Execute shell in running container
docker exec -it <container_id> /bin/sh

# Check Docker disk usage
docker system df

# View build history
docker history <image_name>
```

## Common Issues & Solutions

### 1. Container exits immediately

```bash
# Check exit code
docker inspect <container_id> --format='{{.State.ExitCode}}'

# View last logs
docker logs <container_id>
```

**Fixes**:
- Ensure CMD/ENTRYPOINT runs a foreground process
- Check for missing dependencies
- Verify environment variables

### 2. Build fails

```dockerfile
# Use multi-stage builds for smaller images
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:20-alpine AS runner
WORKDIR /app
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
CMD ["node", "dist/index.js"]
```

### 3. Networking issues

```bash
# List networks
docker network ls

# Inspect network
docker network inspect <network_name>

# Check container IP
docker inspect -f '{{range.NetworkSettings.Networks}}{{.IPAddress}}{{end}}' <container>
```

### 4. Volume permission issues

```dockerfile
# Create non-root user
RUN addgroup -g 1001 -S appgroup && \
    adduser -u 1001 -S appuser -G appgroup

# Set ownership
COPY --chown=appuser:appgroup . .

USER appuser
```

## Dockerfile Best Practices

```dockerfile
# 1. Use specific tags, not :latest
FROM node:20.10-alpine

# 2. Set working directory
WORKDIR /app

# 3. Copy dependency files first (better caching)
COPY package*.json ./
RUN npm ci --only=production

# 4. Copy source after dependencies
COPY . .

# 5. Use non-root user
USER node

From OneWave-AI/claude-skills, MIT.

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