Can a variable be used in docker FROM?

Question

I am wondering if a env variable can be used in a docker from? Reason for this is to control the tagging. For example, say I have this line in my Dockerfile:

From myApp 

What I want is this:

From myApp:${VERSION} 

This way I can say docker build . myApp --build-arg VERSION=9

The process to build docker images for this app is the same. I don't want to have Dockerfiles that are almost identical just to use a different base image.If I want to build version 9, it should use version 9 of the base image.

Answer 1

Quoting this link :

This is now possible if anyone comes here looking for answers: https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact

FROM instructions support variables that are declared by any ARG instructions that occur before the first FROM.

ARG  CODE_VERSION=latest FROM base:${CODE_VERSION} CMD  /code/run-app  FROM extras:${CODE_VERSION} CMD  /code/run-extras 

Answer 2

For at least this docker version this is feasible

docker --version docker version 18.09.8, build bfed4f5 

It requires a preset variable in Dockerfile e.g.

ARG TAG=latest FROM traefik:${TAG} 

Then you can override this preset with the following

docker build --build-arg "TAG=2.2.8" -t my-app:$TAG 

The version number will not show up during build. if you want to test if it works, reference a non-existing version - it will fail with: manifest my-app:version not found.