Push Docker images to Amazon ECR Public Gallery

Many deployments, especially in AWS, prefer pulling images from https://gallery.ecr.aws/ because it's closer to app servers (in AWS) and avoids Docker Hub rate limits.

Searching for "flagsmith" leads to a few results, demonstrating that people have been maintaining their own Flagsmith Docker images in ECR, much likely as an attempt to improve deploying to EKS/ECS in AWS.