Allow Moodle app port to be configured

[NeillM]

Before this change the moodle-app was always put onto port 8100, this makes it impossible (without using local.yml) to have multiple instances of moodle-docker running if they also have a copy of the app running.

After this change while be default nothing should change you may now select the exposed port, or disable it altogether using MOODLE_DOCKER_APP_PORT

Before this change MOODLE_DOCKER_APP_PORT was used internally to select the source port for the app, this is now done using MOODLE_DOCKER_APP_INTERNAL_PORT because that name fits better with the other environment variables that let the user set the port available on the host.

[NeillM]

I ran this both on Linux and Windows using docker images for the app:

• No MOODLE_DOCKER_APP_PORT to check that the app was started on port 8100 as before these changes
• MOODLE_DOCKER_APP_PORT set to 0 to check no mapping happened
• MOODLE_DOCKER_APP_PORT set to a port other than 8100 to check that was picked up

[NeillM]

One of the unit tests failed... since I did not change any Moodle code I'm not sure that is related to this change.

[NeillM]

I have rebased the change and fixed the conflict.

[NeillM]

The Moodle 3.9 failure seems to be something to do with the absence of composer, rather than something to do with this change.

Run export MOODLE_DOCKER_DB="pgsql"
Running: bin/moodle-docker-compose exec -T webserver vendor/bin/phpunit --filter core_dml_testcase
You need to set up the project dependencies using Composer:

    composer install

You can learn all about Composer on [https://getcomposer.org/.](https://getcomposer.org/)
Error: Process completed with exit code 1.