Docker containers have revolutionized the way applications are built and deployed. But with this flexibility comes the responsibility of securing container environments against vulnerabilities, misconfigurations, and runtime exploits.

This guide walks through best practices for Docker container security, including image hardening, runtime protection, and vulnerability scanning, ensuring your workloads remain secure across development and production.

Why Docker Security Matters

Containers are isolated but not inherently secure. A single misconfigured Dockerfile or outdated base image can lead to:

  • Privilege escalations
  • Data breaches
  • Lateral movement across clusters
  • Exploitable CVEs in production

Hardening and scanning should be integral parts of your DevSecOps pipeline.

1. Use Minimal and Verified Base Images

Start with a minimal base image to reduce the attack surface:

  • Prefer alpine, scratch, or vendor-verified images
  • Avoid latest tags—use pinned version numbers
FROM python:3.11-slim

# Instead of:
# FROM ubuntu:latest

Verify image authenticity using Docker Content Trust (DCT):

export DOCKER_CONTENT_TRUST=1

2. Apply the Principle of Least Privilege

Avoid running as root inside the container:

RUN adduser --disabled-password --gecos '' appuser
USER appuser

Additionally, use USER directives and runtime flags:

docker run --user 1001 --read-only myapp:secure

3. Scan Images for Vulnerabilities

Use tools to catch CVEs and outdated packages:

  • Trivy
  • Grype
  • Docker Scout
  • Snyk

Example using Trivy:

trivy image myapp:latest

Automate scanning in your CI/CD pipeline:

- name: Scan Docker Image
  run: trivy image myapp:latest

4. Reduce Image Size and Layer Footprint

Fewer layers and packages mean fewer vulnerabilities:

  • Combine commands into a single layer
  • Remove unnecessary tools and caches
RUN apt-get update && apt-get install -y \
curl && \
apt-get clean && \
rm -rf /var/lib/apt/lists/*

5. Limit Capabilities and Use Seccomp Profiles

Drop all Linux capabilities and only add what’s needed:

docker run --cap-drop ALL --cap-add NET_BIND_SERVICE myapp:secure

Apply seccomp and AppArmor profiles for syscall filtering:

docker run --security-opt seccomp=seccomp-profile.json myapp:secure

Use docker info to verify default security profiles are applied.

6. Avoid Secrets in Images

Do not embed credentials or API keys in your Dockerfiles or images.

Instead, use:

  • Environment variables from secret stores
  • Docker Secrets (Swarm) or Kubernetes Secrets
  • Runtime secret injection (e.g., HashiCorp Vault, AWS Secrets Manager)

7. Use Read-Only File Systems

Prevent unwanted writes during runtime:

docker run --read-only -v /tmp myapp:secure

Only mount specific volumes as writable if required.

8. Enable Container Runtime Protection

Use security tools to monitor running containers:

  • Falco: Detects anomalous behavior from syscalls
  • AppArmor / SELinux: Mandatory access controls
  • Runtime security agents: e.g., Sysdig Secure, Aqua Trivy, Prisma Cloud

These help detect and stop unexpected file access, privilege changes, or network activity.

9. Regularly Patch and Rebuild Images

Set a policy to rebuild containers with updated dependencies:

  • Monitor base image CVEs
  • Use tools like Dependabot, Renovate, or Watchtower
  • Rebuild even if app code hasn’t changed

10. Isolate Containers with Network Policies

Avoid open inter-container communication:

  • Use --network none when networking isn’t needed
  • Define custom user-defined bridges or overlay networks
  • In Kubernetes: Apply Network Policies using Calico or Cilium

Conclusion

Container security starts at build time but must extend through the entire lifecycle. By applying these Docker security best practices—from hardened builds and least privilege to vulnerability scanning and runtime detection—you can protect your applications from a wide range of threats.

In a world of rapid deployments and ephemeral infrastructure, secure-by-default containers are your first line of defense.