Add support for PORT environment variable

[faustinoaq]

Description of the Change

Add support for PORT environment variable, 3000 is used by default.

port: (ENV["PORT"]? || 3000).to_i

Alternate Designs

No

Benefits

• Allow to use heroku-buildpack-amber
• Allow to change PORT easier on services with random port like Heroku

Possible Drawbacks

No, if PORT isn't defined port 3000 is used

[faustinoaq]

I just realize this PR isn't needed, because now we are using config/application.cr to overwrite environment files, so we just need to fix PORT documentation here (Currently this has a comment in wrong place, see: settings.process_count vs settings.port )

[elorest]

This was removed recently by @eliasjpr as env.yml's no longer support dynamic code. I'll submit a PR that fixes this now.

[faustinoaq]

@elorest Yeah, I was using amber v0.3.6 so I didn't notice this change

I think avoiding dynamic code in yaml files is ok, let me close this 👍

[elorest]

#429

Please review this!