feat: Use PORT environment variable as default port

[BrunnerLivio]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #125

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[sinedied]

@BrunnerLivio Would it be possible to update this so that it can be merged?

[sinedied]

@micalevisk I have an up-to-date version of almost the same changes here, but an extra improvement: #672

I've credited @BrunnerLivio and he even said to go with my PR instead of this stale one, could you then reopen it?

[micalevisk]

@sinedied I cannot reopen PRs. Didn't saw that one. Looks better