Describe the bug

While investigating the work done by @ahukkanen in #9597, i have noticed that the port of the PR cannot be changed. While investigating further, i came across the below snippet, which uses a wrong ENV variable.

As per documentation, the correct variable should be PORT.
Also, if the environment variable, the UrlOptionResolver should rely on the config.action_controller.default_url_options

References:

• Change the custom public port ENV variable name to HTTP_PORT #9598
• Fix pipeline asset absolute URLs #9597

To Reproduce
Steps to reproduce the behavior:

1. make sure you are on a branch taht contains the fix for Fix pipeline asset absolute URLs #9597
2. Open the terminal and type in PORT=3301 bundle exec rails c
3. in the console type in Decidim::UserPresenter.new(Decidim::User.first).avatar_url
4. See that the generated port has remained 3000
5. Repeat steps 2 by using HTTP_PORT=3301 bundle exec rails c
6. Repeat steps 3, 4 and observe the port has been changed to 3301

Expected behavior
When using the documented variable for PORT, i should be able to change the URL. Also, any rails default way of customizing the URL should work.

Extra data (please complete the following information):

• Device: [e.g. iPhone6, Desktop]
• Device OS: [e.g. iOS8.1, Windows 10]
• Browser: [e.g. Chrome, Firefox, Safari]
• Decidim Version: [e.g. 0.10]
• Decidim installation: [e.g. Metadecidim]

Additional context
Add any other context about the problem here. For instance, add Metadecidim link.