ci: publish docker image for system tests

[MilanGarnier]

Description

Adds a CI job at dd-trace-php/.gitlab/generate-package.php which pushes the build to github's container registry (similarly to what already exists for prod builds). This is necessary to enable LIBRARY_TARGET_BRANCH to work for the PHP target in utils/scripts/load-binary.sh. (mimicking what already exists for dd-trace-dotnet)

The endgoal is to have DataDog/system-tests#7209 picking up the right branch

Reviewer checklist

[datadog-datadog-prod-us1]

 

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 13 Pipeline jobs failed

DataDog/apm-reliability/dd-trace-php | PHP language tests: [8.2, amd64, zts]     

DataDog/apm-reliability/dd-trace-php | test_extension_ci: [7.0]     

DataDog/apm-reliability/dd-trace-php | test_extension_ci: [8.3]     

View all 13 failed jobs.

⌛ 1 Test performance regression detected

testUserLoginSuccessEvent from tests/Integrations/Laravel/V5_7.DDTrace\Tests\Integrations\Laravel\V5_7\AutomatedLoginEventsTest — 3.67s (+3.1s, +542%)  

ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 54.08% (+0.00%)

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[realFlowControl]

I just recently learned that https://github.com/DataDog/public-images exists, could this do the syncing of the image for us?

[MilanGarnier]

@realFlowControl I didn't know publish-images either, but from what I understand, the tool seems to be centered aroung publishing releases across various sources, whereas what I want to do here is to push once a build, exclusively for system-tests to use. (but I'm really not an expert here)