Machinery for versioned releases

Script to prepare and publish versioned releases.

Workflow:

```
emacs CHANGELOG.md # add entry at top for v0.1.0
git tag -a v0.1.0
git push --tags upstream
./bin/release build
./bin/release publish
```

Perhaps introduce a helper `show_version <container> <image>` so we don't have two copies of this logic.