Convert Dockerfile to Multi-Stage & Update .gitignore

Decrease Dockefile image from 1.46GB to 325MB. Speed up the build by leveraging Docker Multi-Stage technique and using python:3.7-slim-stretch as final image.

This change ignores the docker generated data.

Use jsDelivr for MDI for better global speed

This is extremely useful for mainland Chinese users.

Also fix default value for privacy option of board and pin.

Ref: https://stackoverflow.com/questions/12009423/what-does-status-canceled-for-a-resource-mean-in-chrome-developer-tools

Update spelling of "pinned."

close #50 #115 search boards and pins by tag

close  #52 private pin and board

Now the private pin should only be viewed by owner.

Refactor: Remove unused origin field from Pin

close #163 refactor/docker rework