Fix code scanning alert - Ensure ECR Image Tags are immutable

[dragonfleas]

Tracking issue for:

• https://github.com/synapsestudios/terraform-aws-ecr-repository/security/code-scanning/2

Looking for discussion on this topic, is it important that we use MUTABLE tags? I can't recall a time where we've needed to mutate an already existing tag name, also feel like it's a bad practice.

The security risk there too is if someone gained access to ECR they would be able to swap an image with the same tag for a malicious image.

Tagging @synapsestudios/platform

[spruce-bruce]

Does the reuse of the "latest" tag count as mutable? We do use the latest tag, and we probably need to keep using it