Push/pull task docker image from DockerHub (public & private)

[daigotanaka]

Right now, the Docker image is built on the local machine and pushed to AWS Elastic Container Registry. Then the task pulls the image from there before executing it.

We wish to be able to use DockerHub's public and private repository as well.

Reference:

• https://aws.amazon.com/blogs/containers/authenticating-with-docker-hub-for-aws-container-services/