7 Dockerfile Mistakes That Are Quietly Costing You

Most Dockerfiles work. That's the problem — "it builds and runs" hides a lot of quiet costs in security, speed, and size that don't announce themselves until an audit, an incident, or a cloud bill does it for them. Here are seven mistakes I see constantly, and what to do instead. 1. Running as root By default, the process in your container runs as root — and if someone breaks out, they're root on a surface they shouldn't be. Add a non-root user and switch to it: RUN useradd --system --uid 10001 appuser USER appuser Cheap, and it closes off a whole category of "well, at least it wasn't root" incidents. 2. FROM some-image:latest latest is not a version — it's "whatever was newest when this happened to build." Two builds a week apart can produce different images with no diff to explain it, and a surprise base upgrade is a fun way to spend a Friday. Pin a specific tag, ideally by digest: FROM node:20.11.1-slim 3. Baking secrets into layers COPY .env . or ARG API_KEY followed by using it — and now the secret lives in an image layer forever , recoverable by anyone who pulls the image, even if a later layer deletes the file. Layers are immutable and additive; you can't delete your way out of a leak. Use build secrets ( --mount=type=secret ) or inject at runtime, never at build. 4. No .dockerignore Without one, COPY . . sweeps your .git directory, local env files, node_modules , and test data into the build context — bloating the image and, worse, potentially baking credentials and history into a layer. A five-line .dockerignore is one of the highest-leverage files in the repo. 5. Layer order that destroys your cache COPY . . RUN npm ci # ← reinstalls on EVERY code change Docker invalidates every layer after the first change. Copy the lockfile and install dependencies before copying the rest of your source, so a one-line code change doesn't trigger a full reinstall. This is a build-speed bug hiding as a style choice. 6. Leaving package manager cruft in the image RUN apt-get