Upgrade to alpine:3.9 and add the missing dependencies

[gcauchon]

fixes #70

elixir:1.7-alpine was updated to erlang:21-alpine which is now based on alpine:3.9. A few dependencies now needs to be explicitly added with apk to the runtime Alpine image.

Result

> docker-compose up -d postgresql
Creating network "accent_default" with the default driver
Creating accent-postgres ... done
> docker-compose up application
accent-postgres is up-to-date
Recreating accent ... done
Attaching to accent
accent         | Starting dependencies…
accent         | Starting repos…
accent         | Running migrations for accent
accent         | 18:14:35.060 [info] Already up
accent         | Running seed script for accent
accent         | Success!
accent         | WARNING: Node v10.14.2 is not tested against Ember CLI on your platform. We recommend that you use the most-recent "Active LTS" version of Node.js. See https://git.io/v7S5n for details.
accent         | Could not start watchman
accent         | Visit https://ember-cli.com/user-guide/#watchman for more info.
accent         | 18:14:37.854 [info] Running Accent.Endpoint with cowboy 2.6.1 at http://localhost:4000