Limit CFLAGS suppression to PHP 8.1 only

[bobstrecansky]

Summary

• Scopes the -Wno-error=incompatible-pointer-types CFLAGS to PHP 8.1 only, where the fopencookie signature mismatch is a known issue
• Removes the unconditional ENV CFLAGS from Dockerfile.alpine and conditionally sets it in the build step for both alpine and debian
• Addresses review feedback from Add CFLAGS environment variable to Dockerfile #280

Test plan

• Verify PHP 8.1 builds succeed on both alpine and debian
• Verify PHP 8.2+ builds are unaffected (no unnecessary suppression)

🤖 Generated with Claude Code

[linux-foundation-easycla]

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