[IMPROVEMENT] Refactor and optimize Dockerfile

[AhmedYasserrr]

In raising this pull request, I confirm the following (please check boxes):

• I have read and understood the contributors guide.
• I have checked that another pull request for this purpose does not exist.
• I have considered, and confirmed that this submission will be valuable to others.
• I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
• I give this submission freely, and claim no ownership to its content.
• I have mentioned this change in the changelog.

My familiarity with the project is as follows (check one):

• I have never used CCExtractor.
• I have used CCExtractor just a couple of times.
• I absolutely love CCExtractor, but have not contributed previously.
• I am an active contributor to CCExtractor.

---

Changes:

• Replaced cd commands with WORKDIR instructions to improve clarity and follow Docker best practices.
• Removed the unused RUN export for LIB_CLANG_PATH, which only affected a single build layer (since export does not persist between Docker layers); the library is automatically detected during build.
• Parallelized GPAC build using make -j$(nproc), significantly reducing build time.
• Removed redundant CMD instruction since ENTRYPOINT already defines the execution command.

This PR improves Dockerfile structure, reduces build time, and eliminates unnecessary instructions for a cleaner and more maintainable setup.

[Copilot]

Pull Request Overview

This PR refactors and optimizes the Dockerfile to improve clarity, reduce build time, and remove unnecessary instructions.

• Replaces cd commands with WORKDIR
• Removes an unused RUN export instruction
• Parallelizes the GPAC build with make -j$(nproc) and removes a redundant CMD instruction

Files not reviewed (1)

• docker/dockerfile: Language not supported