ci: add github action docker push

[tommilligan]

Closes #178

Adds Github Actions to:

• Check that the docker image can be built for each new PR
• Build and push a tagged docker image when a new git tag is pushed to the repo (will tag appropriately for tags of format x0.0.0)

This PR also adds installation of node-gyp to the docker image, as I was unable to make the build work otherwise.