Optimize Dockerfile-aws with multi-stage build

[fyliu]

Overview

Refactor Dockerfile-aws to use a multi-stage build to reduce the final production image size. This removes build-time dependencies and temporary files that don't need to be in the final image.

Action Items

• Refactor app/Dockerfile-aws to use a multi-stage build:
  • Stage 1: Builder stage that installs dependencies and runs collectstatic
  • Stage 2: Runtime stage that copies only what's needed from the builder
• Remove build artifacts and cache from the final image
• Compare final image sizes before and after optimization
• Test that the optimized image builds successfully and the app runs correctly
• Document any image size improvements

Acceptance Criteria

• Multi-stage build is implemented in Dockerfile-aws
• Final image size is smaller than the single-stage version
• App starts and serves static files correctly in the optimized image
• Build process completes without errors
• Image size comparison is documented (in commit message or PR description)

References

• Docker multi-stage builds
• We're not sure how much this will reduce the image size, since we already have method to remove build caches from the image.

Parent Issue: #219
Blocked by: #219 sub-issue "Add gunicorn and Whitenoise, and set up Dockerfile-aws and entrypoint-aws.sh"