fix: Use correct kernel headers path for Alpine Linux

Alpine's linux-lts-dev package installs headers to /usr/src/linux-headers-lts/
not /usr/src/linux-headers-<version>. This fixes the kernel module build
failure in the deployment workflow.

Fixes kernel module compilation error:
make[1]: *** /usr/src/linux-headers-6.6.109: No such file or directory

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

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

Dockerfile

@@ -44,8 +44,7 @@ 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 KERNEL_VERSION=$(cat /build/kernel_version.txt) && \
-    make KERNELDIR="/usr/src/linux-headers-$KERNEL_VERSION" || \
+RUN make KERNELDIR="/usr/src/linux-headers-lts" || \
     echo "Kernel module build failed, will use userspace fallback"
 
 # Prepare module for installation