Infrastructure & Operations › Containers

Dockerfile

The recipe for building an image.

Also known as: Dockerfile instructions, docker build

A Dockerfile is a text file of instructions that docker build follows to create an image. Each instruction adds a layer.

FROM python:3.12-slim          # start from a base image
WORKDIR /app                   # set the working directory
COPY requirements.txt .        # copy only the dependency list first
RUN pip install --no-cache-dir -r requirements.txt
COPY . .                       # then copy the rest of the code
EXPOSE 8000                    # documents the port; doesn't publish it
CMD ["python", "-m", "myapp"]  # default command when the container starts

docker build -t myapp:1.0 .    # build from the Dockerfile in this folder
docker run --rm -p 8000:8000 myapp:1.0

The main instructions: FROM (base image), COPY (add files), RUN (run a command while building), ENV (set variables), CMD and ENTRYPOINT (what runs at start; see entrypoint vs CMD).

The classic mistake: bad ordering

Docker caches each layer and reuses it if nothing above it changed. If you COPY . . before installing dependencies, every code edit invalidates the cache and reinstalls everything. Copying the dependency file first, installing, then copying the code keeps rebuilds fast.

Other habits

• Pin the base image version (python:3.12-slim, not latest) for repeatable builds.
• Keep secrets out. Anything copied or set during the build stays in the image’s layers, even if you delete it in a later step.
• Use a .dockerignore file to leave out .git, node_modules and local env files.
• Don’t run as root. See non-root containers.
• Smaller images build, ship and start faster. See multi-stage builds and minimal base images.