add docker-ci target. ref #152

[andreineculau]

• Fixes: #152
• Breaking change: [ ]

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with this PR, running make docker-ci will start a docker container in the same fashion as we do on Travis CI, except for exposing the same environment variables set via webui in Travis CI

how to test:

• you can check out the local-docker-ci branch in support-firecloud and run make docker-ci
• in a repo of your choice that has support-firecloud as a submodule, cd support-firecloud; git fetch: git co origin/local-docker-ci; cd ..; make docker-ci

you might see many warnings, since the script tries to reproduce your local env (same username, same groups) in the docker-ci and there's a mismatch between your local env on MacOS and the docker-ci env (on Ubuntu), but it should be good enough.

[andreineculau]

SF_TRAVIS_DOCKER_IMAGE could maybe be sourced in Makefile from .ci.sh

[andreineculau]

TODO: need to debug group names with whitespaces