Use path based SHELL in Makefiles. It was suggested that this is a better solution for make for cases when there is no /usr/bin/env. See: https://github.com/purpleidea/mgmt/pull/694#discussion_r1015596204
- New docker command for quickly running tasks in a Linux environment. - Updated docs with macOS specific details. - Fixed some test issues. - Add (fallible) macOS test target for Travis.