Building Arguments in Docker

Dockerfile's ARG instruction allows you to define variables, which can be used during the image build process. These variables are called build arguments, and they can be used to customize the build process, making it more dynamic and flexible.

To define a build argument, use the ARG instruction in your Dockerfile:

ARG MY_VARIABLE=default_value

The build argument can then be used in various instructions within the Dockerfile. For example:

FROM node:14
ARG MY_VARIABLE
RUN echo "The value of MY_VARIABLE is: $MY_VARIABLE"

To pass a value for the build argument during the build process, use the --build-arg flag with the docker build command:

$ docker build --build-arg MY_VARIABLE=my_value -t my_image .

ENV vs. ARG

Both ARG and ENV instructions are used to define variables in a Dockerfile. However, there are some key differences between them:

• ARG variables are only available during the build process, while ENV variables persist in the resulting image and are available during container runtime.
• ARG variables can be overridden during the build process with the --build-arg flag, while ENV variables can be overridden during container runtime with the -e flag or through environment files.

Parameterizing Docker Builds

Build arguments provide an efficient way to parameterize your Docker builds. This can be especially useful when you need to build images with different configurations or when you want to avoid hardcoding values in your Dockerfile.

For example, you can parameterize the base image version or the installation of additional packages:

ARG BASE_IMAGE_VERSION=latest
FROM node:${BASE_IMAGE_VERSION}

ARG ADDITIONAL_PACKAGES=""
RUN apt-get update && apt-get install -y $ADDITIONAL_PACKAGES

Multi-stage Builds with Arguments

Multi-stage builds allow you to optimize your Docker images by using separate build stages to produce a final, minimal image. Build arguments can be used across different stages to customize the build process.

Here's an example of a multi-stage build with build arguments:

ARG BASE_IMAGE_VERSION=latest
FROM node:${BASE_IMAGE_VERSION} as build
ARG MY_VARIABLE
WORKDIR /app
COPY . .
RUN npm install && npm run build

FROM node:${BASE_IMAGE_VERSION}-slim as production
ARG MY_VARIABLE
WORKDIR /app
COPY --from=build /app/dist /app/dist
RUN echo "The value of MY_VARIABLE in the production stage is: $MY_VARIABLE"
CMD ["npm", "start"]

In this example, we use the MY_VARIABLE build argument in both the build and production stages. This demonstrates how build arguments can be utilized across different stages of a multi-stage build to maintain consistency and avoid code duplication.