MIx environment variables from kubernetes deployment and docker image - kubernetes

Question

I am trying to pass an environment variable in my deployment that should define a prefix based on a version number:
env:
- name: INDEX_PREFIX
value: myapp-$(VERSION)
$(VERSION) is not defined in my deployment but is set in the docker image used by the pod.
I tried to use both $() and ${} but VERSION is not interpolated in the environment of my pod. In my pod shell doing export TEST=myapp-${VERSION} does work though.
Is there any way to achieve what I am looking for? ie setting an environment variable in my deployment that reference an environment variable set in the docker image?

Answer 1

VERSION is an environment variable of the docker image. So you can assign it a value either inside the container or by passing
env:
- name : VERSION
value : YOUR-VALUE
In your case, VERSION is either set by a script inside the docker container or in the Dockerfile.
You can do :
In the Dockerfile, adding ENV INDEX_PREFIX myapp-${VERSION}
Adding a script to your entrypoint as
export INDEX_PREFIX=myapp-${VERSION}
In case you can't modify Dockerfile, you can try to :
Get the image entrypoint file from the docker image (ie: /IMAGE-entrypoint.sh) and the image args(ie: IMAGE-ARGS). you can use docker inspect IMAGE.
Override the container command and args in the pod spec using a script.
command:
- '/bin/sh'
args:
- '-c'
- |
set -e
set -x
export INDEX_PREFIX=myapp-${VERSION}
IMAGE-entrypoint.sh IMAGE-ARGS
k8s documentation : https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/
Hope it could help you.