Inject universal /slim/init instead of requiring container-specific setup

slim now works with any Containerfile by injecting distro-agnostic scripts
at build time:

- /slim/init: mounts special filesystems, configures networking (static
  QEMU slirp), sets up cgroup2, timezone, and rootless container prereqs.
  Parses slim.cmd=<base64> from the kernel cmdline for runtime overrides,
  otherwise execs /slim/exec.

- /slim/exec: generated from the image's CMD/ENTRYPOINT (via podman image
  inspect), overridable via --cmd at build and run time.

Changes:
- New src/inject.rs: inspect image config, infer command, generate
  /slim/exec script, inject /slim/ into mounted rootfs
- New src/scripts/slim-init.sh: the universal init script (include_str!)
- build.rs: rename --init to --cmd, inject scripts before packing,
  drop Meta::save, restructure to ensure unmount always runs
- qemu.rs: hardcode init=/slim/init, add --cmd (base64 on cmdline),
  drop Meta::load, add -no-reboot
- Remove src/meta.rs and meta.toml (no longer needed)
- Cargo.toml: add serde_json + base64, remove unused walkdir + cpio + toml
- Example Containerfiles simplified to plain FROM + CMD
- New example/test.sh for manual verification
- README updated for new workflow

src/build.rs

@@ -1,5 +1,5 @@
-//! Build: mount a podman image, extract vmlinuz, and pack the rootfs as a
-//! gzipped cpio initrd in the registry.
+//! Build: mount a podman image, inject /slim/ scripts, and pack the rootfs
+//! as a gzipped cpio initrd or qcow2 disk in the registry.
 
 use anyhow::{Context, Result, bail};
 use clap::{Args, ValueEnum};
@@ -11,7 +11,7 @@ use std::process::{Command, Stdio};
 use tempfile::NamedTempFile;
 
 use crate::command::cmd;
-use crate::meta::Meta;
+use crate::inject;
 use crate::registry::registry_dir;
 
 #[derive(Args, Debug)]
@@ -21,10 +21,10 @@ pub struct BuildCmd {
     /// Image tag / registry subdir name
     name: String,
 
-    /// The path to the `init` program for the VM.
-    // TODO: infer init based on `COMMAND`/`ENTRYPOINT`?
-    #[clap(long, default_value = "/sbin/init")]
-    init: String,
+    /// Override the command to exec in the VM. Inferred from the image's
+    /// CMD/ENTRYPOINT if not provided.
+    #[clap(long)]
+    cmd: Option<String>,
 }
 
 #[derive(ValueEnum, Clone, Debug)]
@@ -33,16 +33,18 @@ enum ImageKind {
     Initrd,
 }
 
-pub(crate) fn build(BuildCmd { kind, name, init }: BuildCmd) -> Result<()> {
+pub(crate) fn build(
+    BuildCmd {
+        kind,
+        name,
+        cmd: cmd_override,
+    }: BuildCmd,
+) -> Result<()> {
     let image = &name;
     let mount_path = mount_image(image)?;
     println!("Mounted at: {}", mount_path.display());
 
-    // Keep the build result so the image is unmounted even when the build fails.
-    let result = match kind {
-        ImageKind::Initrd => build_initrd(image, &mount_path),
-        ImageKind::Qcow2 => build_qcow2(image, &mount_path),
-    };
+    let result = build_inner(&kind, image, &mount_path, cmd_override);
 
     let unmounted = cmd(&[
         "podman", "unshare", "--", "podman", "image", "unmount", image,
@@ -54,11 +56,29 @@ pub(crate) fn build(BuildCmd { kind, name, init }: BuildCmd) -> Result<()> {
         eprintln!("warning: failed to unmount image '{}'", image);
     }
 
-    Meta { init }.save(image)?;
-
     result
 }
 
+fn build_inner(
+    kind: &ImageKind,
+    image: &str,
+    mount_path: &Path,
+    cmd: Option<String>,
+) -> Result<()> {
+    let config = inject::inspect_config(image)?;
+    let command = cmd.unwrap_or_else(|| inject::infer_command(&config));
+    let env = inject::env(&config);
+    let working_dir = inject::working_dir(&config);
+    let exec_script = inject::build_exec_script(&command, &env, working_dir);
+    inject::inject(mount_path, &exec_script)?;
+    println!("Injected /slim/ (init + exec)");
+
+    match kind {
+        ImageKind::Initrd => build_initrd(image, mount_path),
+        ImageKind::Qcow2 => build_qcow2(image, mount_path),
+    }
+}
+
 fn mount_image(image: &str) -> Result<PathBuf> {
     let mount_path = cmd(&["podman", "unshare", "--", "podman", "image", "mount", image])?;
     Ok(PathBuf::from(mount_path.trim()))
@@ -134,7 +154,7 @@ fn build_initrd(image: &str, mount_path: &Path) -> Result<()> {
     // Pack the rootfs as a gzipped newc cpio archive. The cpio pipeline
     // runs inside the namespace, its stdout is compressed here.
     let initrd_path = reg_dir.join("initrd");
-    let script = r#"cd "$1" && find . -not -path ./vmlinuz | cpio -o -H newc"#;
+    let script = r#"cd "$1" && find . | cpio -o -H newc"#;
     let mut cpio = Command::new("podman")
         .args(["unshare", "--", "sh", "-c", script, "sh", mount_path])
         .stdout(Stdio::piped())