Simplify CI with composite actions and new image tagging approach (#1283)

- Refactors CI to use composite actions so that there's less duplication in the workflow yaml files.
- Switches to a different image tagging approach for the Docker build image: now the git hash of the `tools/` directory is the image tag rather than the commit hash of `HEAD`. This will result in more image reuse if the tools haven't changed, and makes the logic for deciding whether a new image must be built much simpler.
- When running CI on main, ensures the Docker build image is built and uploaded to ECR before running the CI tests.