EFS volume not mounted on Docker container on Elastic Beanstalk

Question

I'm unable to mount an EFS volume on my Docker container running on Elastic Beanstalk. Here's what I have in Dockerrun.aws.json:

{
  "AWSEBDockerrunVersion": "1",
  "Volumes": [
    {
      "HostDirectory": "/mydir",
      "ContainerDirectory": "/mydir"
    }
  ]
}

Here's what I have in .ebextensions/storage-efs-mountfilesystem.config. The rest is the same as this.

option_settings:
  aws:elasticbeanstalk:application:environment:
    EFS_VOLUME_ID: 'fs-*****'
    EFS_MOUNT_DIR: '/mydir'

When I ssh into my Elastic Beanstalk environment, I see that the volume is mounted on the host:

[ec2-user@ip-****** app]$ df
Filesystem                                                  1K-blocks    Used        Available Use% Mounted on
us-east-1a.fs-******.efs.us-east-1.amazonaws.com:/ 9007199254740992       0 9007199254740992   0% /mydir

But in my Docker container, the filesystem is not EFS:

root@99046bd6a358:/reo# df
Filesystem                       1K-blocks    Used Available Use% Mounted on
/dev/xvda1                         8125880 1552688   6472924  20% /mydir

I do not have any VOLUME instructions in my Dockerfile.

Furthermore, if I created a file in my Docker container, that file does not exist when I go back to the host directory. Therefore, /mydir is not coming from the hosts volume. What else do I need to do access my EFS volume on Docker?