Add nginx.conf to Kubernetes cluster

You can create a ConfigMap object and then mount the values as files where you need them:

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-config
data:
  nginx.conf: |
    your config
    comes here
    like this
  other.conf: |
    second file
    contents

And in you pod spec:

spec:
  containers:
    - name: nginx
      image: nginx
      volumeMounts:
        - name: nginx-config
          mountPath: /etc/nginx/nginx.conf
          subPath: nginx.conf
        - name: other.conf
          mountPath: /etc/nginx/other.conf
          subPath: other.conf
  volumes:
    - name: nginx-config
      configMap:
        name: nginx-config

(Take note of the duplication of the filename in mountPath and using the exact same subPath; same as bind mounting files.)

For more information about ConfigMap see:
https://kubernetes.io/docs/user-guide/configmap/

Note: A container using a ConfigMap as a subPath volume will not receive ConfigMap updates.