Docker Issues & Fixes

Container exits immediately

• Symptom: Run container, it stops right away.
• Cause: Main process ended (e.g., CMD script finishes).
• Fix: Use docker run -it for interactive, or ensure long-running process is in CMD.

Port binding not working

• Symptom: Container is running, but service not reachable.

• Causes:

  • Forgot -p host:container mapping.
  • Firewall blocking port.

• Fix: Run with docker run -p 8080:80 ... and check firewall.

Image too large

• Symptom: Built image is hundreds of MB.

• Causes:

  • Using heavy base image (e.g., ubuntu).
  • Not cleaning up apt cache.

• Fix: Use slim images (alpine, debian-slim).
  Example:

  RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*
  

Cannot connect to Docker daemon

• Symptom: docker ps gives permission error.

• Causes:

  • Docker daemon not running.
  • User not in docker group.

• Fix:

  sudo systemctl start docker
  sudo usermod -aG docker $USER
  

Volume data missing after restart

• Symptom: Data lost after container removed.
• Cause: Used bind mount incorrectly or forgot named volume.
• Fix: Use -v mydata:/path/in/container.