title: Kubernetes deployment read-only filesystem error
body: <p>I am facing an error while deploying Airflow on Kubernetes (precisely this version of Airflow <a href="https://github.com/puckel/docker-airflow/blob/1.8.1/Dockerfile" rel="noreferrer">https://github.com/puckel/docker-airflow/blob/1.8.1/Dockerfile</a>) regarding writing permissions onto the filesystem.</p>
  
  <p>The error displayed on the logs of the pod is:</p>
  
  <pre><code>sed: couldn't open temporary file /usr/local/airflow/sed18bPUH: Read-only file system
  sed: -e expression #1, char 131: unterminated `s' command
  sed: -e expression #1, char 118: unterminated `s' command
  Initialize database...
  sed: couldn't open temporary file /usr/local/airflow/sedouxZBL: Read-only file system
  Traceback (most recent call last):
    File "/usr/local/lib/python2.7/dist-packages/airflow/configuration.py", line 769, in
      ....
      with open(TEST_CONFIG_FILE, 'w') as f:
  IOError: [Errno 30] Read-only file system: '/usr/local/airflow/unittests.cfg'
  </code></pre>
  
  <p>It seems that the filesystem is read-only but I do not understand why it is. I am not sure if it is a <strong>Kubernetes misconfiguration</strong> (do I need a special RBAC for pods ? No idea) or if it is a problem with the <strong>Dockerfile</strong>. </p>
  
  <p>The deployment file looks like the following:</p>
  
  <pre><code>apiVersion: extensions/v1beta1
  kind: Deployment
  metadata:
    name: airflow
    namespace: test
  spec:
    replicas: 1
    revisionHistoryLimit: 3
    strategy:
      type: RollingUpdate
      rollingUpdate:
        maxUnavailable: 0
        maxSurge: 1
    template:
      metadata:
        labels:
          app: airflow
      spec:
        restartPolicy: Always
        containers:
        - name: webserver
          image: davideberdin/docker-airflow:0.0.4
          imagePullPolicy: Always
          resources:
            limits:
              cpu: 1
              memory: 1Gi
            requests:
              cpu: 50m
              memory: 128Mi
          securityContext:  #does not have any effect
            runAsUser: 0    #does not have any effect
          ports:
          - name: airflow-web
            containerPort: 8080
          args: ["webserver"]
          volumeMounts:
            - name: airflow-config-volume
              mountPath: /usr/local/airflow
              readOnly: false #does not have any effect
            - name: airflow-logs
              mountPath: /usr/local/logs
              readOnly: false #does not have any effect
        volumes:
        - name: airflow-config-volume
          secret:
            secretName: airflow-config-secret
        - name: airflow-parameters-volume
          secret:
            secretName: airflow-parameters-secret
        - name: airflow-logs
          emptyDir: {}
  </code></pre>
  
  <p>Any idea how I can make the filesystem writable? The container is running as <strong>USER airflow</strong> but I think that this user has root privileges.</p>