feat(gitlab-ci): Jira IN-194. Build one container image.
- Change gitlab pipeline to build and push the docker image once instead of 48 times
- Change the dockerfile to allow us to build a multiarch image
- Publish the finished image to the Cayenne Public Gallery
- Update the deploy pipeline to deploy tags to staging automatically but deploy to prod manually
NOTE: The deploy pipeline which will try to run when this is merged will fail because there is some corresponding terraform config which I need to apply first. I'll get the deployment progressed through the environments and apply the terraform config changes.