Use --build-arg value in Dockerfile FROM parameter

Question

I'm currently using AWS ECR to host my project images and I need the parameter FROM to be dynamic at Dockerfile according to --build-arg sent from the command line. Example:

$ docker build --build-args region=us-east-1 .

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

${region} never gets replaced and I get an error saying the image doesn't exist.

If I RUN echo ${region} it works, the problem seems to be with FROM instruction.

Is there any way to achieve that?

Answer 1

Updated answer:

With the introduction of multi-stage builds, docker added the ability to provide an ARG before the FROM line. This will look like:

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

Note that ARG's are namespaced. When defined before a FROM line, they are available to the FROM lines. And within a stage, they are available until the end of that stage. If you need to use an ARG in multiple namespaces, you can define it multiple times.

---

Original answer from before the introduction of multi-stage builds:

This is not supported as you've found, and as documented in the variable expansion limitations:

Environment variables are supported by the following list of instructions in the Dockerfile:

• ADD
• COPY
• ENV
• EXPOSE
• LABEL
• USER
• WORKDIR
• VOLUME
• STOPSIGNAL

as well as:

• ONBUILD (when combined with one of the supported instructions above)

To workaround this, I suspect you'll need to generate your Dockerfile with an external process first, e.g. with a sed "s/__region__/$region/" <Dockerfile.in >Dockerfile command.