Can a variable be used in docker FROM?
DockerDocker Problem Overview
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:${VERSION}
This way I can say docker build . myApp --build-arg VERSION=9
The process to build docker images for this app is the same. I don't want to have Dockerfiles that are almost identical just to use a different base image.If I want to build version 9, it should use version 9 of the base image.
Docker Solutions
Solution 1 - Docker
Quoting this link :
> This is now possible if anyone comes here looking for answers: https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact
FROM
instructions support variables that are declared by any ARG
instructions that occur before the first FROM
.
ARG CODE_VERSION=latest
FROM base:${CODE_VERSION}
CMD /code/run-app
FROM extras:${CODE_VERSION}
CMD /code/run-extras
Solution 2 - Docker
For at least this docker version this is feasible
docker --version
docker version 18.09.8, build bfed4f5
It requires a preset variable in Dockerfile e.g.
ARG TAG=latest
FROM traefik:${TAG}
Then you can override this preset with the following
docker build --build-arg "TAG=2.2.8" -t my-app:$TAG
The version number will not show up during build. if you want to test if it works, reference a non-existing version - it will fail with: manifest my-app:version not found.
Solution 3 - Docker
You could simply generate your Dockerfile from a template. Put something like this in a Makefile:
MYTAG=latest
.PHONY: Dockerfile
Dockerfile: Dockerfile.in
sed 's/MYTAG/$(MYTAG)/' $< > $@ || rm -f $@
Then you can run:
make MYTAG=8; docker build -t my-app-8 .
This would only make sense if you are frequently building images that
require a different tag in the FROM
line.
Solution 4 - Docker
It is not possible.
Although, you can use a variable tag like from myApp:latest
and overwrite the latest
tag when you're creating a new version.
Solution 5 - Docker
Build your container programatically using buildah (It can take Dockerfile too).
So for your use-case:
VERSION=v0.1.0
myCon=$(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
Solution 6 - Docker
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