fix: Use correct Alpine kernel headers path with package revision

Alpine's linux-lts-dev installs headers to:
/usr/src/linux-headers-<version>-<revision>-lts

Example: /usr/src/linux-headers-6.12.50-0-lts
         /usr/src/linux-headers-6.6.109-0-lts

The pattern is: /usr/src/linux-headers-${VERSION}-0-lts

Verified on Alpine 3.22 with kernel 6.12.50-0-lts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Dockerfile

@@ -44,7 +44,8 @@ RUN git clone https://github.com/amnezia-vpn/amneziawg-linux-kernel-module.git
 
 # Build the kernel module
 WORKDIR /build/amneziawg-linux-kernel-module/src
-RUN make KERNELDIR="/usr/src/linux-headers-lts" || \
+RUN KERNEL_VERSION=$(cat /build/kernel_version.txt) && \
+    make KERNELDIR="/usr/src/linux-headers-${KERNEL_VERSION}-0-lts" || \
     echo "Kernel module build failed, will use userspace fallback"
 
 # Prepare module for installation