Use docker-container-action with build-args

[yinchi]

This implements my suggested change in #57. An ARG line is added to the Dockerfile to represent the Sphinx version. The action.yml file is edited to use Docker Container Action as GitHub's own offering lacks the required build-args feature.

A new input sphinx-tag is added, defaulting to "2.4.4" for compatibility with the current master behavior.

Note that the synchronize workflow should no longer be needed, nor the dozens of tags in this repo.