Publish datetimestamp tags to docker hub on merge to master branch

[princerachit]

Is your feature request related to a problem? Please describe

Fixes to the images are not published when they are merged to the master branch.
They are released after X days in order to avoid updating latest tag too often.
Users don't have access to the fixed images unless they are released.

Describe the behaviour you'd like

When a fix is merged to the master branch then only datetime stamp images should be released to dockerhub.

Describe alternatives you've considered

We initially went with an approach of releasing new tags including latest and datetimestamp manually, preferably, once a week. This did not work well so we are making this change now.

Another idea we explored was to have something like preview tag that gets pushed along with every merge to master branch. However, we want to avoid adding more tags so we did not go ahead with this idea.

Additional context

The default image:tag used by gipod is workspace-full:latest, we cache the latest image to our cluster in order to speed up image pulls. However, if we release latest tag to often then this cache becomes useless for us.

[princerachit]

Some work was done in this direction before #730