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

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.

Answer 2

You could simply generate your Dockerfile from a template. Put something like this in a Makefile:

MYTAG=latest

.PHONY: Dockerfile
Dockerfile: Dockerfile.in
    sed 's/MYTAG/$(MYTAG)/' $< > $@ || rm -f $@

Then you can run:

make MYTAG=8; docker build -t my-app-8 .

This would only make sense if you are frequently building images that require a different tag in the FROM line.

Answer 3

It is not possible.

Although, you can use a variable tag like from myApp:latest and overwrite the latest tag when you're creating a new version.

Answer 4

Build your container programatically using buildah (It can take Dockerfile too).

So for your use-case:

 VERSION=v0.1.0

 myCon=$(buildah from myApp:${VERSION})

 buildah config --cmd "sleep 1d" $myCon

 buildah commit $myCon $USER/sleeping1d

You can obviously script it, save and invoke it, and one more advantage is buildah doesn't need docker daemon running, which is great for CI. Also it's an open-source project, check out the project page.

BTW I saw this issue lately which is exactly what you want - https://github.com/projectatomic/buildah/issues/581

Answer 5

Unfortunately it's not possible to do that. The first line of your Dockerfile must be a FROM directive, and so that precludes the use of the ARG directive. There is a good answer there from larsks about generating a Dockerfile, but I'd also like to suggest merely creating different Dockerfiles and then specifying a particular one in your docker build command using the -f switch:

docker build -t codemiester/app:latest -f ./Dockerfile.apache2.ubuntu