Add production Alpine and Debian Stretch Docker images (#225)

Add production Alpine and Debian Stretch Docker images (#225)

* First pass at a (swarm-compatible) docker image

Uses an existing traefik server as a https reverse proxy.

* Add example config for a Docker swarm deployment

* Point to traefik config for docker compose setup

* Clarify that traefik is needed for the example config

* Use node:8-stretch base image and don't install yarn

(The base image already contains yarn.)

* Initial commit for an Alpine Docker image

* Fix docker volume path

* Merge #213 and #225 and move files around

* Remove unnecessary dependencies from the alpine build

* Update Dockerfiles to match install path, config path, etc.

* Update the configuration in the example compose file

* Update the configuration in the example swarm file

* Remove the declared networks and volumes from the compose example, which are not strictly required

* Update attachment path in the documentation

* Display traefik as a suggestion and not a required dependency

* Update the Docker ignored files

* Fix typos reported in #225

* Move production Dockerfiles to a production directory

* Add the redis configuration settings

* Add Docker files to the dockerignore

* Make the signup limit configurable