6.3 Container Runtime Config

Container runtime config is the complete set of settings applied to a container at the moment it is created and run — environment variables, resource limits, network configuration, restart policy, mounted volumes — collectively determining exactly how that container behaves once running, distinct from anything baked into the underlying image.

The Layered Relationship Between Image and Runtime Config

An image provides defaults — default environment variables, a default command, a default working directory — while runtime configuration can override or supplement nearly all of these defaults at the moment a specific container is created.

ENV LOG_LEVEL=info
CMD ["node", "server.js"]

docker run -e LOG_LEVEL=debug myapp:1.0

The runtime configuration here overrides the image's default LOG_LEVEL, demonstrating how the same image can behave differently across different container instances depending on their specific runtime configuration.

Inspecting a Container's Actual, Effective Configuration

The complete, effective configuration a running container is actually operating under — combining image defaults with any runtime overrides — can be inspected directly.

docker inspect myapp --format '{{json .Config}}'

Why Centralizing Runtime Config Matters for Consistency

For deployments managing many containers, capturing runtime configuration explicitly (in a script, a Compose file, or an orchestrator's configuration) rather than relying on manually remembered command-line flags ensures consistent, reproducible container behavior across multiple deployments.

services:
  app:
    image: myapp:1.0
    environment:
      - LOG_LEVEL=debug
    restart: unless-stopped

Why Understanding Container Runtime Config Matters

A clear understanding of exactly what runtime configuration controls, and how it interacts with an image's own defaults, is essential for confidently customizing container behavior without unexpected surprises arising from a misunderstood interaction between the two.