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Dockerfile Image A Dockerfile is a “script for building an image”: one instruction per line, starting from a base image and step by step copying files, installing dependencies, and setting up the environment to produce a reproducible image. This page covers how it relates to the build context and builds one from scratch. It assumes you can already use the basic docker commands (see CLI commands).

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Build context and tag

In docker build -t myapp:1.0 .:
  • -t myapp:1.0: the image tag (name:version, omitting the version defaults to latest).
  • The trailing .: the build context. The builder only sees files inside the context, not paths outside it.
  • -f: specifies the Dockerfile location (defaults to Dockerfile in the context root).
project dir (build context)
├── Dockerfile          ← list of instructions
├── .dockerignore       ← exclusion list
├── app.py
└── requirements.txt

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.dockerignore

Same syntax as .gitignore, sits in the context root, and keeps things out of the builder, shrinking the context and avoiding needless cache invalidation: 
.git
__pycache__
*.pyc
.env
.venv
node_modules
*.md
tmp/
Supports ** and ! (negate to keep). A bigger context ships slower, so .dockerignore is a file worth preparing by default.

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Building a Dockerfile from scratch

1

Pick a base image (FROM)

Start from a suitable base, pinning a version tag rather than latest (see Choosing images):
FROM python:3.12-slim
2

Set the working directory (WORKDIR)

Following COPY / RUN are relative to this; the directory is auto-created if missing:
WORKDIR /app
3

Install dependencies, copy code (COPY + RUN)

Copy the manifest first, install, then copy the source (order matters, see layer cache):
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
4

Set the start command (CMD)

What runs when the container starts:
CMD ["python", "app.py"]
5

Build the image

Run it from the Dockerfile’s directory; . is the build context:
docker build -t myapp:1.0 .
A build (with BuildKit) looks like this, each layer matching an instruction:

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Next

Reference: docs.docker.com/build/concepts/context