title: Combining multiple k8s secrets into an env variable
body: <p>My k8s namespace contains a <code>Secret</code> which is created at deployment time (by <code>svcat</code>), so the values are not known in advance.</p>
  
  <pre><code>apiVersion: v1
  kind: Secret
  type: Opaque
  metadata:
    name: my-database-credentials
  data:
    hostname: ...
    port: ...
    database: ...
    username: ...
    password: ...
  </code></pre>
  
  <p>A <code>Deployment</code> needs to inject these values in a slightly different format:</p>
  
  <pre><code>...
  
  containers:
    env:
    - name: DATABASE_URL
      valueFrom:
        secretKeyRef:
          name: my-database-credentials
          key: jdbc:postgresql:&lt;hostname&gt;:&lt;port&gt;/&lt;database&gt; // ??
  
    - name: DATABASE_USERNAME
      valueFrom:
        secretKeyRef:
          name: my-database-credentials
          key: username
  
    - name: DATABASE_PASSWORD
      valueFrom:
        secretKeyRef:
          name: my-database-credentials
          key: password
  </code></pre>
  
  <p>The <code>DATABASE_URL</code> needs to be composed out of the <code>hostname</code>, <code>port</code>, 'database` from the previously defined secret.</p>
  
  <p>Is there any way to do this composition? </p>