Build Docker images on Pull Requests

[nodiscc]

Building docker images from each Pull Request would ease testing of the proposed changes. Docker users would only have to set the proper image tag in their deployment script/compose file to start testing a development branch of Shaarli. Images could be stored on the free 500MB container registry attached to the project if we want to separate release/official (hub.docker.com) and development/testing images.

Running these builds/pushing to the registry should probably require maintainer approval (it is probably possible to abuse package/image upload permissions from a malicious PR, depending on how we set it up).

It is also required to properly test changes that affect the Docker image build itself #1789 (review).

Related to #1555 (automatic build/upload to the package registry fro full .zip builds)

[nodiscc]

PR at #2014

ease testing of the proposed changes

I have left this aspect aside for now, the new workflow only ensures that the image builds without errors, but does not push to the registry. I think pushing images for every PR is not necessary, and would clutter the registry over time - we would have to manually clean it up from time to time.