Use GitHub Actions instead of Circle CI

[grgur]

I propose we migrate to GH Actions for this repository testing so we can use the GH Bot out of the box instead of personal tokens for commenting.

[mitchellsimoens]

The downside to gimbal's dockerfile actions is the dockerfile targets the gimbal docker image on docker hub... not the local code in the repo. So like if I were to open a pull request here it will run the local code to check the outcome of any changes. You can execute node code so we could execute gimbal via code but the reason why I didn't do that was because the need for browsers.

Now we could maintain a separate dockerfile to include the browser stuff and copy the gimbal build into the image and use npm link to install it globally (or just execute the bin/index.js directly).