[workspace] Add step in the CI for docker images build

[ascariandrea]

Since we have dockerized both yt:backend and tk:backend we may need to add a step in the CI to properly check docker images can be built smoothly.

We also need to push the images to our registry when we release a new version on master

• build tk:backend for any PR
• build yt:backend fro any PR
• push yt:backend on release (on master)
• push tk:backend on release (on master)