Cannot use a base image from ECR in Github Actions

[dancorne]

I've been testing fac/dynamic-thumbnail-generation with a base image stored in ECR, but hoping to move away from it so it's not critical, and can be worked around. However it's unintuitive behaviour...

Setting up a Github Action to assume arn:aws:iam::486229364833:role/allow_ecr_push_pull_access to pull/push images should have authentication to pull a base image from our ECR repos but it returns an error when trying to access it. Unfortunately I can't find my example in the logs now but it was clear it couldn't pull the base image due to authentication issues. I think it works locally though, maybe because Docker auth works separately to the AWS login IIRC.