two-containers-sharing-same-volume-in-a-pod.yaml

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: nginx
  name: nginx-test
  namespace: default
spec:
  containers:
  - image: nginx
    imagePullPolicy: Always
    name: nginx
    volumeMounts:
    - mountPath: /usr/share/nginx/html
      name: shared-volume
  - image: ubuntu
    imagePullPolicy: Always
    name: ubuntu
    command: ["/usr/bin/tail"]
    args: ["-f", "/dev/null"]
    volumeMounts:
    - mountPath: /shared
      name: shared-volume
  volumes:
  - name: shared-volume
    emptyDir: {}

The two containers - nginx and ubuntu - spin up inside the same pod and share a filesystem between them. This is mounted under /shared inside the ubuntu container and /usr/share/nginx/html in the nginx container. If you port forward port 80 to the nginx container and try to web browse to it you will see a 403 because there is no index.html. If you then exec a shell into the ubuntu container and run:

echo "hello" > /shared/index.html

Then reload the page served by nginx, you'll see a web page with the string hello on it. Proving the two containers are sharing a filesystem.