For our use case we store all of our ECR repositories in a single
account, and access these repos from others. Therefore we require the
need to specify the account which possess the repostories, along with
the region.

The region wasn't a necessary change however I have seen others
requesting this and it was an easy implementation alongside this work.

Reason for this is because when trying to grab the region via the means
previously implemented it was unsuccessful on a Buildkite agent running
on EC2.

As we have changed to using `get-login-password` due to the previous
implementation being deprecated, it requires the region as well as the
account id to be given as part of the docker login command.

Ideally I would leave the region out of these changes entirely.

If nothing is given both via the `region` flag or `AWS_DEFAULT_REGION`
env var, it will default to `eu-west-1`.