Set work directory to /opt/phabricator

Summary: Fixes T562.

Test Plan:

docker build --tag nasqueron/phabricator . && \
./run-test-new-container && \
docker exec -it phabtest pwd

Reviewers: dereckson

Projects: Nasqueron Docker deployment squad, Docker images

Maniphest Tasks: T562

Differential Revision: http://devcentral.nasqueron.org/D65