Respect USER directive from OCI image config

Add support for the Dockerfile USER directive so that the container's
CMD/ENTRYPOINT runs as the configured user instead of root.

Changes:
- inject.rs: Add user field to Config struct, write /slim/user at
  build time
- build.rs: Pass config.user through to inject()
- slim-init.sh: Read /slim/user and drop privileges via su before
  executing the command. Numeric uids are resolved to usernames via
  /etc/passwd (BusyBox su does not accept numeric args). When dropping
  privileges, run as a child (not exec) so PID 1 stays root and can
  poweroff after the command exits.
- test.sh: Add test_user verifying build-time CMD and --cmd override
  both run as the configured user

Closes #7

src/inject.rs

@@ -22,6 +22,8 @@ pub struct Config {
     pub env: Vec<String>,
     #[serde(default)]
     pub working_dir: Option<String>,
+    #[serde(default)]
+    pub user: Option<String>,
 }
 
 pub fn inspect_config(image: &str) -> Result<Config> {
@@ -85,33 +87,30 @@ pub fn build_exec_script(command: &str, env: &[String], working_dir: Option<&str
     lines
 }
 
-/// Inject /slim/init and /slim/exec into a mounted container image rootfs.
-pub fn inject(mount_path: &Path, exec_script: &str) -> Result<()> {
+/// Install `content` into the mounted rootfs at `<mount>/slim/<name>` with
+/// the given mode.
+fn install_into_rootfs(mount: &str, name: &str, mode: &str, content: &str) -> Result<()> {
+    let temp = tempfile::NamedTempFile::new()?;
+    fs::write(temp.path(), content)?;
+    let src = temp.path().to_str().context("temp path is not UTF-8")?;
+    let dest = format!("{mount}/slim/{name}");
+    cmd(&[
+        "podman", "unshare", "--", "install", "-D", "-m", mode, src, &dest,
+    ])?;
+    Ok(())
+}
+
+/// Inject /slim/init, /slim/exec, and optionally /slim/user into a mounted
+/// container image rootfs.
+pub fn inject(mount_path: &Path, exec_script: &str, user: Option<&str>) -> Result<()> {
     let mount_str = mount_path.to_str().context("mount path is not UTF-8")?;
 
-    let init_temp = tempfile::NamedTempFile::new()?;
-    let exec_temp = tempfile::NamedTempFile::new()?;
-    fs::write(init_temp.path(), SLIM_INIT)?;
-    fs::write(exec_temp.path(), exec_script)?;
+    install_into_rootfs(mount_str, "init", "755", SLIM_INIT)?;
+    install_into_rootfs(mount_str, "exec", "755", exec_script)?;
 
-    let init_src = init_temp
-        .path()
-        .to_str()
-        .context("temp path is not UTF-8")?;
-    let exec_src = exec_temp
-        .path()
-        .to_str()
-        .context("temp path is not UTF-8")?;
-
-    let init_dest = format!("{mount_str}/slim/init");
-    let exec_dest = format!("{mount_str}/slim/exec");
-
-    cmd(&[
-        "podman", "unshare", "--", "install", "-D", "-m", "755", init_src, &init_dest,
-    ])?;
-    cmd(&[
-        "podman", "unshare", "--", "install", "-D", "-m", "755", exec_src, &exec_dest,
-    ])?;
+    if let Some(user) = user.filter(|u| !u.is_empty()) {
+        install_into_rootfs(mount_str, "user", "644", user)?;
+    }
 
     Ok(())
 }