AWS Elastic Beanstalk 'CannotPullContainerError' for private docker image (multi-instance) - amazon-web-services

Question

I have a single-instance elastic beanstalk environment which runs a docker image which is hosted as a private image on Dockerhub. This works fine. I am trying to create a new multi-container environment which runs the exact same image (plus one other, not icluded in my code example here). In the multi-container environment, I cannot get elastic beanstalk to launch my docker image, I get the following error:
ECS task stopped due to: Task failed to start. (img1_name: img2_name: CannotPullContainerError: Error response from daemon: pull access denied for user/repo, repository does not exist or may require 'docker login': denied: requested access to the resource is denied)
Here is the dockerrun for my single-instance environment:
{
"AWSEBDockerrunVersion": "1",
"Authentication": {
"Bucket": "my_bucket",
"Key": ".dockercfg"
},
"Image": {
"Name": "user/repo:tag",
"Update": "true"
},
"Ports": [
{
"ContainerPort": 5000,
"HostPort": 443
}
],
"Logging": "/var/log/nginx"
}
And here is the .dockerfcg file:
{
"auths": {
"https://index.docker.io/v1/": {
"auth": "my_token"
}
}
}
Again, the above works fine.
My multi-instance dockerrun file is as follows:
{
"AWSEBDockerrunVersion": "2",
"authentication": {
"bucket": "my_bucket",
"key": ".dockercfg"
},
"containerDefinitions": [
{
"name": "img_name",
"image": "user/repo:tag",
"essential": true,
"memoryReservation": 128,
"portMappings": [
{
"hostPort": 80,
"containerPort": 5000
}
]
}
],
"Logging": "/var/log/nginx"
}
I have ssh-ed into my elastic-beanstalk instance and run the following to check that it is able to access the .dockercfg from my s3 bucket:
aws s3api get-object --bucket mybucket --key dockercfg dockercfg
I have also tried various different formats for the .dockercfg file including...
{
"https://index.docker.io/v1/": {
"auth": "zq212MzEXAMPLE7o6T25Dk0i",
"email": "email#example.com"
}
}
{
"auths": {
"https://index.docker.io/v1/": {
"auth": "zq212MzEXAMPLE7o6T25Dk0i"
}
}
}
I'm tearing my hair out over this, I've found a few similar threads here and on AWS forums, but nothing seems to resolve my issue. Any help greatly appreciated.