Improve matching of `Dockerfile` in `.dockerignore`

[cplee]

Allow paths like Dockerfile in .dockerignore to match paths like ./Dockerfile that are passed in as the docker build context.

https://github.com/stelligent/mu/blob/develop/common/docker.go#L103-L104

[shubhambhattar]

@cplee Can you explain this a bit more? I would like to work on this issue.

[cplee]

@shubhambhattar the problem was that if someone adds the string ./Dockerfile to their .dockerignore file, then running mu svc push will fail since the Docker daemon doesn't receive the Dockerfile in its context.

The goal is that the Dockerfile and the .dockerignore should ALWAYS be included in the archive that is sent to Docker daemon.

[jdbohrman]

I'd like to have a crack at figuring this out, still looking for help?

[cplee]

@jdbohrman yes please!