Using --build-arg to modify FROM clause in Docker image

Question

Say we have a Dockerfile like so:

FROM node:9

and we build it with:

docker build -t foo .

my question is - is there a way to change the FROM clause using a --build-arg, something like this:

ARG NODE_VERSION
FROM node:$NODE_VERSION

and the build that with:

docker build -t foo --build-arg NODE_VERSION="8" .

doing this however, would probably destroy any caching ability the image could otherwise do :( — Alexander Mills, Jun 19 '18 at 21:24

score 1 · Accepted Answer · answered Jun 19 '18 at 22:21

1

It works exactly like you have proposed. Given the following Dockerfile:

ARG base_image=alpine
FROM $base_image

I can build it like this and get an Alpine based image:

docker build -t test1 .

Or like this to get a Fedora based image:

docker build -t test2 --build-arg base_image=fedora .

As you say, changing the base image would invalidate the cache.

answered Jun 19 '18 at 22:21

larsks

that is really cool thanks, I was trying `FROM node:$NODE_VERSION`, and it wouldn't accept a variable there. – Alexander Mills Jun 19 '18 at 22:26
so if I do `ARG base_image=alpine`, that means that `alpine` is just the default value, right? – Alexander Mills Jun 19 '18 at 22:27

1 Answers1