Thai's FYP

[thailekha]

Main change

An self-contained endpoint for pushing an image where:

• Dockerhub credentials are retrieved from the body of the request
• The credentials are written into a temporary file, mounted into a docker container
• Dockerhub authentication is done within the docker container, then the image is pushed, and the
  docker` container is removed
• Eventually no authentication data is kept on the Gantry server. Hence, there is no need manage auth state or do login/logout

[thailekha]

Hi @StephenCoady , I created this PR as a reference for my report :)

[StephenCoady]

Hi @thailekha, thanks for the contribution. I will start working through the PR and seeing what we can/can't merge. Excellent additions to the server from a first glance though, thanks!