Allow native Docker deployment via Docker Compose/Docker Stack

[michiels]

We would like to have deployment done via a regular Docker package and docker-compose. This will make deploying and updating Intercity very easy. We currently have the awesome CLI installer made by @jvanbaarsen. But there are lots of moving parts in maintaining that. Migrating to a native Docker package would simplify things.

@ariejan already did some amazing work in #242

Todo

• [ ] Implement script and Docker (Compose) setup to install and start Intercity on production
• [ ] Document ideal installation path in https://github.com/intercity/intercity-next/blob/master/doc/installation.md
• [ ] Figure out an upgrade path to current users
• [ ] Describe upgrade path from current installs in https://github.com/intercity/intercity-next/blob/master/doc/installation.md

[michiels]

This feature has been functionally implemented as first go via #265. It works well and is easily performed by running two shell commands on a server:

$ wget https://raw.githubusercontent.com/intercity/intercity-next/master/scripts/bootstrap.sh
$ bash bootstrap.sh

Now that this works, it can be tested and we can devise an upgrade path for current Intercity users that use the old installation command-line tool. Most importantly is being able to migrate or load the postgres files into the new Docker volume that is used.

Besides that, updating the repository README and installation docs should be done!

[michiels]

Initial new installation method is merged via: https://github.com/intercity/intercity-next/pull/265