Add working-directory input

[Roman Nazarenko]

First of all, great project, very helpful, esp the docker support, thank you!

It'd be great to support setting a custom working directory for the command to be run in. I've spent about an hour making cd subproject/ && sbt test ... work, and had to settle on evaling everything, which is not ideal.

I think it'd get much easier if it could do something like this:

    - name: Build & publish the project
      uses: matankdr/github-docker-sbt
      with:
        command: sbt test docker:publishLocal
        working-directory: projectname-ap # here we go

[Matan Keidar]

Hi @Jtalk , thanks for your feedback!

I think I'll have time to address this issue only next week.
In the meantime, you are more than welcome to send pull request.
Will be happy to assist of course :)

[Roman Nazarenko]

No problem, check the PRs when you can, mate!

I also took a liberty of drawing up one to make the docker-registry input optional. I felt it'd be easier to fast-track it past creating an issue etc. Feel free to reject have you any reasons to require it explicitly defined.