containerizationlisted

# Containerization Containers package the application and its dependencies into a reproducible unit. They answer: "will this run the same way everywhere?" ## Image layering Order matters. Least-frequently-changing layers first (OS packages → language runtime → dependencies → application code). A one-line code change should rebuild only the last layer. ## Multi-stage builds Build in one stage, run in another. The production image contains no compilers, no dev dependencies, no source code — only the artifact and its runtime. ## Image size A 2GB image is a deployment delay. Strip package manager caches, remove temp files, use `--no-install-recommends`. The image is the unit of deployment; smaller images deploy faster and have a smaller attack surface. ## Non-root The container must not run as root. `USER 1000` or equivalent. A compromised process running as root owns the node. ## Health checks The container must report its own health. `HEALTHCHECK` in Docker, `livenessProbe`/`readinessProbe` in Kubernetes. A container that is running but not serving is worse than one that crashed — it looks alive but silently fails.