Docker Build Arguments

Related Posts

• Docker CLI Cheatsheet
• Docker Compose Cheatsheet

What Are Build Arguments?

Before going into build arguments, it is worth clarifying the difference between arguments and environment variables as they overlap quite a lot.

Build arguments are variables for the sole purpose of building your image. Environment variables may be used to in various ways to help you in building your image, but their main purpose is to pass information to your container when it is being run (after it has been built).

This diagram from a post on vsupalov.com explains it best:

Declaring Build Args In The Dockerfile

You can specify ARG as part of your Dockerfile at the start (above the FROM statement), in order to declare the arguments your build needs. These can then be used later as part of the build. E.g.

ARG ACCOUNT_ID

FROM ubuntu:latest
RUN echo $ACCOUNT_ID

Default Value

In the previous example, we required the ACCOUNT_ID to be provided, but we could specify a default value that would be overridden, like so:

ARG ACCOUNT_ID="def"

FROM ubuntu:latest
RUN echo $ACCOUNT_ID

Specifying Build Arguments

In order to provide the build arguments, one can provide them in a variety of ways outlined below:

Docker Build Command

One can specify the build arguments at the point of calling the build like so:

#!/bin/sh
docker build \
  --build-arg ACCOUNT_ID=abc \
  --build-arg SECOND_BUILD_ARG=admin \
...

Docker-compose Command

If using docker-compose build instead of docker build, then one would specify the build arguments in the same manner like so:

docker-compose build \
  --build-arg ACCOUNT_ID=abc \
  --build-arg SECOND_BUILD_ARG=admin

Docker Compose FIle

The build arguments can also be specified in the docker-compose file like so:

version: '3'
services:
  app:
    build:
       context: .
       args:                                                                     
         - ACCOUNT_ID=abcd