Compile FFI from source using AWS Lambda image

[Strech]

What does this PR do?

Switch layer build from ruby:X.Y (Debian) to public.ecr.aws/lambda/ruby:X.Y so native extensions compile against Lambda's actual glibc/libffi.

After datadog gem install, uninstall precompiled FFI (missing Ruby 3.2 ABI) and reinstall from source. Fixes cold-start LoadError on ffi_c with DD_APPSEC_ENABLED=true on Ruby 3.2.

Also adds MAKEFLAGS parallelization and --no-document to gem installs.

Motivation

This should help with datadog gem upgrade in the future.

Testing Guidelines

Locally + CI

Additional Notes

Build time improvement

Step	Before	After
gem install datadog-lambda	~2 min	~2 min
gem install datadog	~6 min	~5 min
gem install ffi	~3 min	~2 min
Total	~11 min	~9 min

This PR is a compilation of #144 and #152

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[zarirhamza]

LGTM - thanks for picking this up