Add GitHub Actions workflow for building the container

[LukeShortCloud]

The container image for Video2X is only built when a new release is created and published on pushes to the "main" and "dev/*" branches. For every commit, docker build should be ran to ensure the container is built successfully.

This allows us to find and fix issues like this sooner:

• https://github.com/k4yt3x/video2x/issues/911
• https://github.com/k4yt3x/video2x/issues/1012

Long-term, we can expand the workflow to run functional tests (for example, upscaling and interpolating with every back-end on a very small video clip). It could even publish a video2x:latest image.