This PR modifies the existing build-sign-upload-dom0-rpm script to use a docker container so that it can be run on macos rather than needing to be a linux machine.

Changes include:

• Dockerfile with necessary dependencies
• Fetching the secret key from secrets manager, fetching the secret ID from parameter store (as this repo is in the open I'm trying to minimise 'private' stuff included in the build scripts)
• A script to handle signing the RPM (this was previously done in the same script but now it needs to be run in a docker container it's easier to have it in a separate file)