env_config.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    run: nginx
  name: nginx
spec:
  replicas: 5
  selector:
    matchLabels:
      run: nginx
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        run: nginx
    spec:
      containers:
      - image: nginx:1.16
        name: nginx
        env:                                #<-- Adding Environment variables to a container
        - name: VIA_SPEC                
          value: "From Spec"                #<-- Value hardcoded in the spec
        - name: VIA_CONFIGMAP
          valueFrom:                        #<-- instead of direct value reference it
            configMapKeyRef:                #<-- all info to get a specific variable value 
              name: nginx-config
              key: example.value

---
apiVersion: v1
kind: ConfigMap                             #<-- defining a configMap
metadata:
  name: nginx-config
data:
  example.value: "From Configmap"           #<-- one of many possible values