Replaced the ubuntu pssh container with an alpine based image.

[rustyrazorblade]

• pssh is now a docker subproject located at docker-pssh.
• The base image is switched from ubuntu to alpine linux
• The beginnings of a build test system are included, but non functional.

You'll need to build the pssh docker container locally to test it. Do the following before you start your review:

./gradlew clean :docker-pssh:buildDocker shadowJar

I'll push it up when merging into master so it'll be available for folks when they run the container.