build: change Dolphin to use mage-OS repository by default

[MateusDadalto]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/graycoreio/mage2docker/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

Dolphin by default uses adobe's repo to install magento. This imply in having to setup a magento account and creating an access key for anyone who want to use dolphin.

Fixes: #66

What is the new behavior?

Change the default repository to be mage-OS. Mage-OS is the open initiative to maintain magento. It provides a Magento mirror that requires no authorization

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[MateusDadalto]

PR is failing because 2.3.7 and magento/module-wishlist-sample-data have dependency conflict. This is happening because we don't have older versions of magento/module-wishlist-sample-data in mage-os mirror

Here's the log for it: https://github.com/graycoreio/mage2docker/runs/7680790648?check_suite_focus=true#step:9:3809

[Vinai]

I'll add the required magento/module-wishlist-sample-data to the mirror repo. Will let you know when it is available.

[Vinai]

The magento/module-wishlist-sample-data package is now available win the version 100.3.5, which has requirements matching Magento 2.3.7-p3.