Add an option to run the `test-unit`, `test-style`, and `test`
steps from the `Makefile` insides of a docker container. Doing so
isolates this component of helm development from any other aspect of
your global go environment.

These commands all have the name `docker-*`. Long term, there may
be reproducibility benefits to running all of the Make steps in a
container by default, in which case `containerized-test-unit` could
become `test-unit`.