Add a preference option to set docker build context path

[nyamakawa]

I have a multiple docker project which has common pip requirements.

Project files are:

• docker
  • local
    • app
      • Dockerfile
  • production
    • app
      • Dockerfile
• requirements
  • requirements.txt

And Dockerfile is like:
FROM python:3.6 COPY ./requirements /requirements/ RUN pip install -r /requirements/requirements.txt ...

I was trying to build image from "Build Image" context menu,
but build was failed on COPY line with some error:

"COPY failed: stat /var/lib/docker/tmp/docker-builder282882071/requirements: no such file or directory"

VSCode generated below command:
docker build local-app -f docker/local/app/Dockerfile docker/local/app/Dockerfile

Since 'docker build' send files below build context path to docker-machine to build image,
and do not see parent directories SO link , I modified command manually :
docker build local-app -f docker/local/app/Dockerfile ./
and this time it is succeeded.

If image build context path can be set from workspace preference, it may be useful.
So I wrote a little patch.
I confirmed this issue in Docker tool box for mac and Mac Docker.

Thank you.

[msftclas]

All CLA requirements met.

[chrisdias]

LGTM