Dockerfile LABEL from executable version - docker

Question

I am trying to label my docker image with my app version from within my Dockerfile using the LABEL command.
I can write the version into a file with the following:
RUN /opt/app/foo --version > /var/app-version
/var/app-version would now contain a plaintext file with the application version inside, ready for use in my label.
Now I need a way to place this into a docker variable/arg such that I can do something like the following:
LABEL application_version=$APP_VERSION
Or perhaps there is another way to achieve the same?

Answer 1

You're not going to be able set a label using content generated dynamically from something installed inside the container. You can set a label dynamically from your host environment using a build argument, like this:
FROM docker.io/alpine:latest
ARG application_version=1
LABEL application_version=${application_version}
If I build the container like this:
docker build -t myimage .
I will have:
$ docker inspect image myimage | jq '.[0].Config.Labels'
{
"application_version": "1"
}
Whereas if I build the container like this:
docker build --build-arg application_version=3 -t myimage .
I will have:
$ docker inspect image myimage | jq '.[0].Config.Labels'
{
"application_version": "3"
}
If you use the same build argument value to select the version of the
software you install in your container, you can ensure that the
installed version and the label match.