How to use a --build-arg parameter before FROM in a Dockerfile

Question

I'm working on AWS ECR for hosting my project images. I'm using the FROM parameter to dynamically set the Dockerfile according to --build-arg input in the CLI.

$ docker build --build-args region=eu-north-1 .

// Dockerfile
FROM aws.ecr.huge.url.${region}/repo:php-apache
WORKDIR /var/www
RUN echo "@@@"

Here I want to replace the ${region} but that never happens and an error show up. But, when I run echo ${region} it works leading me to believe the problem is in FROM instruction

Can someone please help??

Answer 1

In the earlier docker version this wasn't possible, but now after the introdiction of multi-stage builds, the user can provide an argument before FROM. SOmething like this:

# default to eu-north-1
ARG region=eu-north-1
FROM aws.ecr.huge.url.${region}/repo:php-apache

Also, the arguments are namespaced. Therefore, when you define them before a FROM line, they're available to FROM until the end of stage. To use args in multiple namespaces, you'll have to define it every time.