fix: Fix rails Docker setup & postgresql

[xredo]

Reason for change

After running the docker setup, Rails complained during bundle install. Some pg binaries were missing in the alpine image. Also, during Rails init process it crashed because the Alpine OS didn't have the tzdata library. Those errors has been solved adding those dependencies in the base image installation.

Finally, in order to successfully start the Postgres DB we need to define the username and password env variables.

List of Changes

• Change Dockerfile image for Rails application
• Change postgres docker-compose to include default username & password

Checklist

• [x] I have updated necessary documentation and links in the README.md or the doc folder
• [x] I have rebased from master, written good commit messages and squashed unnecessary commits