Allow external stacks. Also default to `stack.yml` if file was not specified.
Allow external stacks.
Also default to `stack.yml` if file was not specified.

polyverse-security/play-with-polyverse,franela/play-with-docker,play-with-docker/play-with-docker,polyverse-security/play-with-polyverse,play-with-docker/play-with-docker,polyverse-security/play-with-polyverse,franela/play-with-docker,polyverse-security/play-with-polyverse,franela/play-with-docker,play-with-docker/play-with-docker,play-with-docker/play-with-docker,franela/play-with-docker