Kubernetes puzzle: Populate environment variable from file (mounted volume) - kubernetes

Question

I have a Pod or Job yaml spec file (I can edit it) and I want to launch it from my local machine (e.g. using kubectl create -f my_spec.yaml)
The spec declares a volume mount. There would be a file in that volume that I want to use as value for an environment variable.
I want to make it so that the volume file contents ends up in the environment variable (without me jumping through hoops by somehow "downloading" the file to my local machine and inserting it in the spec).
P.S. It's obvious how to do that if you have control over the command of the container. But in case of launching arbitrary image, I have no control over the command attribute as I do not know it.
apiVersion: batch/v1
kind: Job
metadata:
generateName: puzzle
spec:
template:
spec:
containers:
- name: main
image: arbitrary-image
env:
- name: my_var
valueFrom: <Contents of /mnt/my_var_value.txt>
volumeMounts:
- name: my-vol
path: /mnt
volumes:
- name: my-vol
persistentVolumeClaim:
claimName: my-pvc

Answer 1

You can create deployment with kubectl endless loop which will constantly poll volume and update configmap from it. After that you can mount created configmap into your pod. It's a little bit hacky but will work and update your configmap automatically. The only requirement is that PV must be ReadWriteMany or ReadOnlyMany (but in that case you can mount it in read-only mode to all pods).
apiVersion: v1
kind: ServiceAccount
metadata:
name: cm-creator
namespace: default
---
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
namespace: default
name: cm-creator
rules:
- apiGroups: [""]
resources: ["configmaps"]
verbs: ["create", "update", "get"]
---
apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
name: cm-creator
namespace: default
subjects:
- kind: User
name: system:serviceaccount:default:cm-creator
apiGroup: rbac.authorization.k8s.io
roleRef:
kind: Role
name: cm-creator
apiGroup: rbac.authorization.k8s.io
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: cm-creator
namespace: default
labels:
app: cm-creator
spec:
replicas: 1
serviceAccountName: cm-creator
selector:
matchLabels:
app: cm-creator
template:
metadata:
labels:
app: cm-creator
spec:
containers:
- name: cm-creator
image: bitnami/kubectl
command:
- /bin/bash
- -c
args:
- while true;
kubectl create cm myconfig --from-file=my_var=/mnt/my_var_value.txt --dry-run -o yaml | kubectl apply -f-;
sleep 60;
done
volumeMounts:
- name: my-vol
path: /mnt
readOnly: true
volumes:
- name: my-vol
persistentVolumeClaim:
claimName: my-pvc