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🆘 FAQ & Troubleshooting ​

Issue 1: Container Exits Immediately After Starting ​

bash
# Step 1: Check the exit code
docker inspect --format='{{.State.ExitCode}}' myapp

# Step 2: Check the logs
docker logs myapp

# Common causes and solutions:
# ExitCode 0   — App finished and exited (likely a misconfigured CMD)
# ExitCode 1   — App error, check the logs
# ExitCode 126 — Command not permitted, check if chmod +x is needed
# ExitCode 127 — Command not found, check PATH
# ExitCode 137 — OOM killed, increase --memory limit
# ExitCode 139 — Segmentation fault, check application code

# Debug with interactive mode
docker run -it --entrypoint /bin/bash myapp
# Manually run the startup command inside to see the error

Issue 2: Container Has No Network Access ​

bash
# Check if container DNS is working
docker exec myapp cat /etc/resolv.conf

# Test if the container can resolve domain names
docker exec myapp nslookup google.com

# Check container network mode
docker inspect --format='{{.HostConfig.NetworkMode}}' myapp

# Reset Docker networks
docker network prune

# If host can't reach container IP in bridge mode — check IP forwarding
sysctl net.ipv4.ip_forward
# Should be 1; if it's 0:
sysctl -w net.ipv4.ip_forward=1
echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf

Issue 3: Docker Eats Up All Disk Space ​

bash
# Check Docker disk usage
docker system df -v

# One-click cleanup (removes stopped containers, unused images, unused networks)
docker system prune -a -f

# Clean up all unused volumes (⚠️ will delete data!)
docker volume prune -f

# Clean up build cache
docker builder prune -a -f

# Configure log rotation (prevent logs from filling disk — the most common cause!)
# /etc/docker/daemon.json
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}
systemctl restart docker

Issue 4: docker pull Is Slow / Times Out ​

bash
# Configure mirror acceleration (essential for users in China)
cat > /etc/docker/daemon.json << 'EOF'
{
  "registry-mirrors": [
    "https://mirror.ccs.tencentyun.com",
    "https://docker.1ms.run"
  ]
}
EOF
systemctl restart docker

# Verify it's in effect
docker info | grep -A 5 "Registry Mirrors"

# If your company has a private registry
docker login registry.company.com
docker pull registry.company.com/myapp:latest

# Manually specify platform for pulling (fixes multi-arch issues)
docker pull --platform linux/amd64 myapp:latest

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