Store Base Images in Docker

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The docker images have been uploaded to the container registry (Docker Hub):

• Images to run the web application and celery in development and production environments: https://hub.docker.com/r/bluexolo/bluexolo
• Images to run Robot Framework (v4.0, v3.2.2 and v2.8) SSH servers: https://hub.docker.com/r/bluexolo/robot
• Image for the web application proxy (just for production environments): https://hub.docker.com/r/bluexolo/proxy
• Image for the assistant (this one was not modified, is available since the release of the chatbot feature): https://hub.docker.com/r/bluexolo/assistant

There are other images required to run BlueXolo, not mentioned in the list because they are on their own official repos, redis:6.2.1-alpine and postgres:9.6.21-alpine specifically.