Five Docker optimizations shrink a 1.2GB image to 180MB without code changes

A software engineer inherited a bloated 1.2GB Docker image causing slow CI builds and lengthy deployments, and reduced it to roughly 180MB using five layered techniques. The biggest gain came from adopting multi-stage builds, which separate the build environment from the runtime image so compilers and dev tools are never shipped in the final container. Switching from the full node:20 base image to the leaner node:20-slim variant cut additional hundreds of megabytes, while also reducing the security attack surface. Reordering Dockerfile instructions to copy dependency lockfiles before source code improved cache efficiency, cutting rebuild times from four minutes to about 20 seconds. Adding a .dockerignore file and combining apt cleanup commands within single RUN layers prevented unnecessary files and cached package data from inflating image size.