Updated Dockerfile

[timothyclarke]

Changes:

• Update the dockerfile to be current
• Switched to a multi stage build (616MB -> 221MB)
• re-pathed (in runtime container) dropping the src as is build
• re-pathed the serving directory as the http sever runs at / where the code was at /qrcode2stl
• Building the code from the current dir rather than cloning master