Store built Layman docker image on docker hub

[index-git]

To shorten the runtime of GitHub Actions (automated tests). As of now 'build' step takes 5 minutes, I expected to shorten it to max of 4 minutes.

[jirik]

It was partially implemented in v1.20.0 by PR #770 (images are suitable only for test environment)

[jirik]

Another step could be to split Dockerfiles into two parts: the first part that does not use env settings and install maximum number of dependencies, and second part that uses env settings. Then we can use first-part Dockerfiles to build reusable dev/demo images and store them at DockerHub.