feat: download alpine linux-virt kernel for single --kernel

README.md

@@ -38,7 +38,7 @@ ecr [OPTIONS] <DISTRO[:VERSION]> [-- COMMAND...]
 | `--no-cache` | Force a fresh download, bypassing the cache |
 | `-v, --verbose` | Print diagnostic output (URLs, layer info, extraction steps) |
 | `-a, --arch <ARCH>` | Target architecture (`amd64`, `arm64`, `armhf`, `riscv64`, …) |
-| `--kernel <PATH>` | Boot with QEMU system emulation using specified kernel |
+| `--kernel [PATH]` | Boot with QEMU system emulation. Downloads Alpine's `linux-virt` kernel if no path provided |
 | `-m, --memory <SIZE>` | Memory for QEMU VM (default: 2G, only with `--kernel`) |
 
 ## Examples
@@ -59,11 +59,14 @@ ecr --arch arm64 alpine -- uname -m
 # Always pull a fresh image
 ecr --no-cache fedora
 
-# Boot with QEMU system emulation (requires qemu-system-<arch>)
+# Boot with QEMU system emulation (auto-downloads default kernel)
+ecr --kernel alpine
+
+# Boot with your own kernel
 ecr --kernel /boot/vmlinuz ubuntu
 
 # Boot with custom memory
-ecr --kernel /boot/vmlinuz --memory 4G alpine
+ecr --kernel --memory 4G alpine
 ```
 
 ## QEMU System Mode
@@ -71,18 +74,23 @@ ecr --kernel /boot/vmlinuz --memory 4G alpine
 When `--kernel` is specified, ecr boots the rootfs in a full QEMU virtual machine instead of using namespaces:
 
 ```sh
-ecr --kernel /boot/vmlinuz alpine
+# Auto-download Alpine's linux-virt kernel (recommended)
+ecr --kernel alpine
+
+# Use your own kernel
+ecr --kernel /boot/vmlinuz ubuntu
 ```
 
 This mode:
 - Creates a gzipped CPIO initramfs from the rootfs
-- Boots QEMU with your kernel
+- Boots QEMU with your kernel (or auto-downloads Alpine's `linux-virt` kernel)
 - Provides full VM isolation
 - Works for any architecture (no binfmt_misc needed)
+- Caches the default kernel in `~/.cache/ecr/`
 
 Requirements:
 - `qemu-system-<arch>` installed
-- Kernel with serial console support
+- For custom kernels: kernel must have serial console support
 
 ## Supported distributions