Add CI/CD

[arm4b]

Docker Hub removed free builds which we used for this repository to automatically build and push the images to https://hub.docker.com/r/stackstorm/st2packs/tags.

Since the feature is not available anymore, add CI/CD with Github Actions to build and push the images to Docker Hub for every update/push to the master.

[arm4b]

The build pipeline in GH Actions would be simple as

• 1. Build docker images (by running make all)
• 2. Push the docker images to the Docker Hub

[MANASAB805]

I’m new to open source and would love to contribute. Could you please assign this issue to me?

Thanks!