Fix docker development image, add env validation and small fixes

[dragonhunt02]

General fixes

• Fixed config/prod.exs not loading (MIX_ENV=prod was not set)
• Interpolate variables in .env files for easier configuration
• Move env variable validation after application startup, since .env variables are not available at build time in Docker.
• Update Quick Setup instructions

Development image fixes

• Remove frontend volume mount that triggered build failure. frontend/ folder in docker image was replaced by docker-compose when mounting, so node modules installed in image were removed.
• Removed frontend entrypoint override that caused failure on image run
• Fix missing depends_on, links causing build failure. In docker-compose files, extends won't extend some properties (see Restrictions section)
• Add development Caddyfile to enable HTTP without TLS server on port 80 for testing
• /mailbox load failed with a 401 error. As a temporary fix it was moved to /api/v1/mailbox path, but this issue needs review.