Add Docker support

[serge1peshcoff]

What version of this package are you using?
Latest I guess?

What problem do you want to solve?
There's no Docker container for instant.io, so it's impossible to run it in a Docker sandbox. This should make the self-hosted installation/deployment really easy, as running the Docker container is really 1 command.

What do you think is the correct solution to this problem?
Add Dockerfile, probably add CI for building and pushing the container on merging to master. I can take care of this if needed.

Are you willing to submit a pull request to implement this change?
Yes

[zackees]

I did it and added it here:
https://github.com/zackees/instant.io

It's being deployed on Render.com and my computer, but should work for DigitalOcean and others.