For others visiting, this now possible using the stringData field of a Secret definition.

So for this example, I have a secret called environment-secrets. This secret contains a single key called env.yml, which is itself a YAML file containing a number of key=value parings. I want to mount this secret onto my pods at /credentials/env/env.yml.

Firstly I define the object definition for the secret as follows in a file called environment-secret.yml:

- kind: Secret
  apiVersion: v1
  metadata:
    name: environment-secret
  type: Opaque
  stringData:
    env.yml: |-
      db_host=my-db.host
      db_user=db-user
      db_password=db-pass

I then create the secret using kubectl create -f environment-secret.yml. Upon creation, the values in the env.yml stringData field are converted into a env.yml entry in the data field of the secret.

Next I update my PodSpec to mount the secret at my desired location. So I firstly define the secret as a volume:

volumes:
   - name: environment-secret
     secret:
     optional: false
      secretName: environment-secret

And then update the mountPoints to mount this secret at my preferred location within my containers:

volumeMounts:
- mountPath: /credentials/env
  name: environment-secret

After this I have a file on my container at /credentials/env/env.yml that contains my key=value pairings.