feat: Add PCOV setup to Dockerfile

[kofimokome]

On Mymo, installing PCOV in the Dockerfile does not work.
When running tests, PHP can't find pcov.

This PR solves the problem by configuring PCOV directly in the image.

[sourceant]

Code Review Summary

The PR adds PCOV support to both the main and package Dockerfiles. This is a common requirement for high-performance PHP unit test coverage.

🚀 Key Improvements

• Enabled PCOV extension via PECL to fix issues where PHP couldn't find the extension during tests.

💡 Minor Suggestions

• Consolidate RUN commands to reduce image layers and clean up PECL cache.

[sourceant]

Review complete. No specific code suggestions were generated. See the overview comment for a summary.