'exec user process caused: exec format error' in AWS Fargate Service

Short answer: docker buildx build --platform=linux/amd64 -t <image-name> .

I think you've identified your problem.

You're building your images on Apple's M1 chip, which is an ARM architecture. Fargate is probably running on the more common Intel x86-64 architecture. Images you build locally on your Mac aren't going to be able to run there.

The easiest solution is probably to have your images build automatically in Docker Hub (or use a Github Action to build them in Github).

I don't have all the details about how you're building and deploying your images, so it's possible I'm missing some details.