How Docker layer caching works and why instruction order matters in Dockerfiles

Each instruction in a Dockerfile that modifies the filesystem creates a read-only layer, and Docker reuses cached layers during rebuilds as long as inputs remain unchanged. Once any layer changes, all subsequent layers are rebuilt from scratch, making instruction order critical for build performance. A common mistake is copying all source code before installing dependencies, which forces package managers like pip to reinstall on every minor code change. The recommended fix is to copy and install the dependency manifest first, then copy application code, so the slower install step is only re-executed when dependencies actually change. This principle applies broadly across ecosystems, including Node.js, Go, and Ruby projects.