Fix docker hub auth, pull image on build

[odracci]

In this PR I fixed:

• DockerHub Auth
• Pull image defined in FROM directive in Dockerfile
• Fix sha global variable
• Add Login link in the frontend
• Refactor Dockerfile to have less layers and cache npm install

[odracci]

@odino I tested it with docker hub but not with a private registry

[odino]

Would you mind trying it out on a regular registry? You can use this dumb repo to try it out locally :)

[odracci]

@odino It works 😄

[odino]

I'll take your word on it :)

[odino]

BTW @odracci we have switched to Google Cloud Container Builder :) https://cloud.google.com/container-builder/docs/ still use roger here and there but most images are now built on GCP.