Use Public ECR Gallery, instead of dockerhub to avoid rate limit

[opp-svega]

We recently started to use self hosted runners and are running into issues with Docker's rate limit. After reviewing the repo, would it be possible to use another docker repository that is a little more forgiving?

The Dockerfile references the following images:
composer:2.5.7 and php:8.1.9-alpine

We can get similar images from AWS ECR's Public Gallery, which has a rate limit for unauthenticated pulls at 1 per second, instead of Dockerhub's 100 per 6 hours.

public.ecr.aws/docker/library/composer:2.5.7
public.ecr.aws/docker/library/php:8.1.9-alpine

Let me know what you think, if approved, i can make a PR.