Pass image from smoke to deploy, restore build caching

[HebaruSan]

Motivation

Currently we build the Inflator image twice when pushing to master, once for smoke testing and again for pushing to Docker Hub. This is a small but not tiny amount of extra time and repeated work.

Another few extra seconds are spent re-downloading build dependencies that formerly were cached in the old workflows.

Changes

• The Inflator image build / push / redeploy steps are moved out of build.cake and into smoke.yml and deploy.yml using the standard docker actions and commands, in order to allow the steps to be separated and smoke testing inserted before push.
  https://github.com/docker/build-push-action
• smoke.yml now uploads a tar of the image as an artifact after the smoke test passes
  https://docs.docker.com/build/ci/github-actions/share-image-jobs/
  https://docs.docker.com/build/ci/github-actions/test-before-push/
• deploy.yml now downloads the tar of the image and loads it instead of building it
• The caching of the tools and dependencies is restored in build.yml

This should eliminate some redundant work.

[HebaruSan]

Two learnings:

1. It seems to still rebuild the image between load and push. I guess the export doesn't include caching.
2. The redeploy commands need AWS_REGION rather than AWS_DEFAULT_REGION 😵‍💫
   Oops, no. We needed to tell docker run to make AWS_DEFAULT_REGION available inside the container with -e.