shellspec unit tests can fail due to unpredictable dev environment

[cameronmeissner]

Shellspec unit tests currently use whatever shell happens to be installed on the developer's machine.

The problem with this is that the scripts shellspec is supposed to be testing are specifically meant to be run with a bash shell. This means that if a developer is using a different shell, such as zsh, shellspec tests will end up failing in any case where the scripts being tested utilize shell syntax that is not exportable from bash.

Solution would be to have shellspec tests run within a container where we can control exactly which shell is available within the environment.