Dockerfile Overview
Docker builds images by reading the instructions from a Dockerfile. A Dockerfile is a text file containing instructions for building your source code.
Filename
The default filename is Dockerfile. When the default filename is used, when you run docker build, the Dockerfile is utilized.
Docker Images
Docker images consist of layers. Each layer is the result of a build instruction in the Dockerfile. Layers are stacked sequentially, and each one is a delta representing the changes applied to the previous layer.
Example
# Instructs the docer builder what version
# syntax to use.
# `docker/dockerfile:1` points to the
# latest release of the version 1 syntax.
# syntax=docker/dockerfile:1
# Specify the OS:VERSION used in the Docker image
# There are public images available at the Docker Hub.
# Local images may also be used.
FROM ubuntu:22.04
# install app dependencies
RUN apt-get update && apt-get install -y python3 python3-pip
RUN pip install flask==3.0.*
# install app
# By doing this, `hello.py` is added to the build context of
# the Docker image. The build context is the set of files
# that the build can access.
COPY hello.py /
# final configuration
# If the application uses enviornment variables, you can
# set them this way.
ENV FLASK_APP=hello
# Mark that the final image has a service listening on port 8000
EXPOSE 8000
# Start the application
# The `CMD` instruction sets the command that is run when the
# user starts a container based on this image.
CMD ["flask", "run", "--host", "0.0.0.0", "--port", "8000"]
Building
To build the container image using the Dockerfile created previously, you would run the command
docker build -t test:latest .
The -t test:latest option specifies the name and tag of the image.
The single dot at the end of the command sets the build context to the current directory. This means that the hello.py file needs to be in the same directory as to where tho command is being invoked.
After the image has been built, the application can be run as a container with
docker run -p 127.0.0.1:8000:8000 test:latest
This publishes the container's port 8000 to https://localhost:8000 on the Docker host.
Common Instructions
| Instruction | Description |
|---|---|
FROM <image> |
Defines a base for your image. |
RUN <command> |
Executes any commands in a new layer on top of the current image and commits the result. RUN also has a shell form for running commands. |
WORKDIR <directory> |
Sets the working directory for any RUN, CMD, ENTRYPOINT, COPY, and ADD instructions that follow it in the Dockerfile. |
COPY <src> <dest> |
Copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>. |
CMD <command> |
Lets you define the default program that is run once you start the container based on this image. Each Dockerfile only has one CMD, and only the last CMD instance is respected when multiple exist. |