Add CI build

[arcusfelis]

This repo historically use two stage build, before Docker stages were built-in feature. There is some info how to use it in README. Except, no one-line-command to use it (so, we can test building locally).

This PR adds this one-liner as a ./build.sh script. Be aware, it is not called when we build MongooseIM images in the main repo. But to ensure that script is actually working, I've added CI task into this repo.

Also, added a couple of tweaks to make life easier.

Changes:

• Add github actions simple test script so we don't break something.
• Reorder commands, so deps are installed once (probably useless for circle-ci, but ok for local dev)
• Add mongooseim into path, so we don't need to use long paths when exec into a container.
• Use CMD instead of ENTRYPOINT.
• Execute mongooseim bootstrap.
• Add a smoke test.