Refactor build.sh a bit

[jshufro]

1. Removes the requirement that build.sh must be run from the project's parent dir
2. Ignores build/ subtree
3. Cleans up artifact naming a bit to reduce complexity of build docker files and remove a bunch of mv from the script
4. Only pushes to docker hub if -u is passed. (allows for local development with the 'official' build pipeline artifacts)