Provide the official container image

[comuttun]

please merge #18 and #19 before this

This fixes #16 .

Implemented ./sharedocs package:docker to build the official docker container image, and implemented ./sharedocs publish:docker to push the official docker container image to Dockerhub.

In addition, modified .travis.yml to build/push the official docker container image automatically.

Note: To push the docker container image successfully, configuring env variable DOCKER_USERNAME and DOCKER_PASSWORD are required on TravisCI.

The official container image repository is sharedocs/sharedocs-standalone, so changed docker-compose.yml to refer that.