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Dockerfile

A Dockerfile is a plain-text file containing instructions that tell Docker how to build an image. Think of it as a recipe:

  • You start from a base image
  • You install software or copy files into it
  • You define how the container should run

FROM

We can use the FROM instruction to start our new image from a known base image. This should be the first line of our Dockerfile.

FROM ubuntu:24.04

Base images typically take the form os:version. Avoid using the latest version; it is hard to track where it came from and the identity of latest can change.

RUN

We can install updates, install new software, or download code to our image by running commands with the RUN instruction.

RUN apt-get update
RUN apt-get upgrade -y
RUN apt-get install -y python3

Each RUN instruction creates an intermediate image (called a layer). Too many layers makes the Docker image less performant, and makes building less efficient. We can minimize the number of layers by combining the RUN instructions:

RUN apt-get update && apt-get upgrade -y && apt-get install -y python3

COPY

There are a couple different ways to get your source code inside the image. One way is to use a RUN instruction with wget to pull your code from the web. When you are developing, however, it is usually more practical to copy code in from the Docker build context using the COPY instruction.

COPY qe-7.4.1-ReleasePack.tar /opt/qe-7.4.1-ReleasePack.tar

ENV

Another useful instruction is the ENV instruction. This allows the image developer to set environment variables inside the container runtime. For example, we usually need to set PATH or LD_LIBRARY_PATH.

ENV PATH="/opt/myprogram/bin:$PATH"

WORKDIR

The WORKDIR instruction in a Dockerfile sets the current working directory for any subsequent instructions like RUN and COPY.

WORKDIR /app

Complete Example

# Step 1: Base image
FROM python:3.11-slim

# Step 2: Set working directory
WORKDIR /app

# Step 3: Copy files into the image
COPY requirements.txt .
COPY app.py .

# Step 4: Install dependencies
RUN pip install --no-cache-dir -r requirements.txt

# Step 5: Run the app
CMD ["python", "app.py"]

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