Previous release script wasn't committing/tagging in the right order.
Also made scripts resistant to:
* empty env vars
* current workdir the script is invoked from
cc: @m-okeefe
Closes#75.
### Changelog
Adds 4 scripts to the `hack/` directory for building/pushing images, injecting images tags into static manifests, and tagging new releases. See [hack/README.md](https://github.com/m-okeefe/microservices-demo/tree/release-script/hack).
**Note**: since we have not pushed images yet, the images in the `./release/` manifests are still set to the skaffold defaults (eg. `adservice`).
