Refactor and add commands to bind to webui/cluster

The one semi-significant downside of this refactor is that
rethinkdb:start no longer reuses containers, as each start is
likely to have different binding characteristics. Ho hum.
Fixes, refactors; static-bind ports at start time
Restore -o pipefail with new method to get ID via inspect
Fix syntax errors
Fix VOLUME_DIR logic

Granted, it's still a little wonky, and could probably use a little
more division in meaning, but this works.
Fix port binding
Fix port bindings EVEN MORE
Fix static port saving
Fix unquoted arg
Run rethinkdb as dokku, not root