Container Security Beyond the Basics

Container images seem straightforward until you’ve debugged a vulnerability in a base image you don’t control. Security at the container layer requires attention most developers skip.

Base Image Selection

Your base image choice cascades through everything you build on top. Alpine is minimal but uses musl instead of glibc, which breaks some software. Distroless images are even smaller but harder to debug. Standard distributions are larger but familiar.

Whatever you choose, pin to specific versions. Pulling “latest” means your builds aren’t reproducible. That vulnerability scan that passed yesterday might fail tomorrow because the base changed.

Layer Optimization

Each instruction in a Dockerfile creates a layer. More layers mean larger images and longer pulls. Combine commands where sensible, but don’t sacrifice readability for marginal size savings.

Order matters for cache efficiency. Put instructions that change infrequently early. Dependency installation should come before application code copy. This way, dependency layers cache even when code changes.

Vulnerability Scanning

Integrate scanning into your CI pipeline. Trivy, Snyk, and built-in registry scanners catch known vulnerabilities before deployment. Block pushes that fail critical thresholds.

Scanning finds problems but doesn’t fix them. You need a process for updating base images when vulnerabilities are discovered. Automated dependency updates help but require testing.

Runtime Security

Don’t run as root inside containers. Create a non-root user and switch to it. This limits damage if the container is compromised.

Drop capabilities you don’t need. The default Linux capability set is overly permissive. Most containers need far fewer privileges than they receive by default.

Use read-only root filesystems where possible. If your application doesn’t write to the filesystem, preventing writes blocks many attack techniques.

Registry Management

Your container registry is as critical as your source repository. Enable scanning, implement access controls, and sign images for verification.

Clean up old images. Registries accumulate cruft quickly. Implement retention policies that keep recent versions but delete ancient ones.