Support host:guest mount syntax with WORKINGDIR resolution

--mount now accepts <host-path>:<guest-path> (Docker-style). When the
guest path is omitted, the host path is used. Non-absolute guest paths
are resolved against the image WORKINGDIR, which is written to
/slim/workdir at build time and read by slim-init.sh at boot.

Changes:
- qemu.rs: Parse host:guest spec, pass guest path (not host path) on
  the kernel cmdline as slim.mount=<tag>:<base64(guest_path)>
- inject.rs: Accept working_dir param, write /slim/workdir into rootfs
- build.rs: Pass config.working_dir to inject()
- slim-init.sh: Read /slim/workdir, resolve relative guest paths
  against it before mounting
- test.sh: Test host:guest absolute paths (multi-mount) and relative
  guest path resolved against WORKINGDIR

Addresses PR #9 review comment from @hulthe.

example/test.sh

@@ -103,6 +103,39 @@ EOF
     rm -rf "$work"
 }
 
+test_user() {
+    echo "=== Testing USER directive ==="
+
+    work="$(mktemp -d)"
+    img="slim-test-user"
+
+    cat > "$work/Containerfile" <<'EOF'
+FROM alpine:latest
+RUN adduser -D -u 1500 testuser
+USER testuser
+CMD ["/bin/sh", "-c", "echo USER_BUILD_OK:$(id -u):$(whoami); poweroff -f"]
+EOF
+
+    echo "-- Building container image with USER directive..."
+    if ! podman build --network=none -t "$img" -f "$work/Containerfile" >/dev/null 2>&1; then
+        report fail "USER directive (podman build failed)"
+        rm -rf "$work"
+        return
+    fi
+
+    echo "-- Test 1: build-time CMD runs as USER"
+    "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1
+    output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" 2>&1 || true)
+    check_output "$output" "USER_BUILD_OK:1500:testuser" "USER build-time CMD runs as testuser"
+
+    echo "-- Test 2: run --cmd override runs as USER"
+    output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" --cmd 'echo USER_RUN_OK:$(id -u):$(whoami); poweroff -f' 2>&1 || true)
+    check_output "$output" "USER_RUN_OK:1500:testuser" "USER run --cmd override runs as testuser"
+
+    cleanup "$img" "$img"
+    rm -rf "$work"
+}
+
 test_mount() {
     echo "=== Testing --mount (9p shares) ==="
 
@@ -268,6 +301,8 @@ cargo build 2>&1
 test_distro "alpine" "alpine:latest" ""
 test_distro "archlinux" "archlinux:latest" "RUN pacman -Sy --noconfirm iproute2 wget; pacman -Sc --noconfirm"
 
+test_user
+
 test_mount
 
 test_service

src/build.rs

@@ -78,7 +78,12 @@ fn build_inner(
     let command = cmd.unwrap_or_else(|| inject::infer_command(&config));
     let exec_script =
         inject::build_exec_script(&command, &config.env, config.working_dir.as_deref());
-    inject::inject(mount_path, &exec_script, config.working_dir.as_deref())?;
+    inject::inject(
+        mount_path,
+        &exec_script,
+        config.user.as_deref(),
+        config.working_dir.as_deref(),
+    )?;
     println!("Injected /slim/ (init + exec)");
 
     match kind {

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> {
@@ -85,54 +87,38 @@ pub fn build_exec_script(command: &str, env: &[String], working_dir: Option<&str
     lines
 }
 
-/// Inject /slim/init and /slim/exec (and optionally /slim/workdir) into a
-/// mounted container image rootfs.
-pub fn inject(mount_path: &Path, exec_script: &str, working_dir: Option<&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 and /slim/workdir
+/// into a mounted container image rootfs.
+pub fn inject(
+    mount_path: &Path,
+    exec_script: &str,
+    user: Option<&str>,
+    working_dir: 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)?;
+    }
 
     if let Some(dir) = working_dir.filter(|d| !d.is_empty()) {
-        let workdir_temp = tempfile::NamedTempFile::new()?;
-        fs::write(workdir_temp.path(), dir)?;
-        let workdir_src = workdir_temp
-            .path()
-            .to_str()
-            .context("temp path is not UTF-8")?;
-        let workdir_dest = format!("{mount_str}/slim/workdir");
-        cmd(&[
-            "podman",
-            "unshare",
-            "--",
-            "install",
-            "-D",
-            "-m",
-            "644",
-            workdir_src,
-            &workdir_dest,
-        ])?;
+        install_into_rootfs(mount_str, "workdir", "644", dir)?;
     }
 
     Ok(())

src/scripts/slim-init.sh

@@ -70,6 +70,39 @@ for tok in $(cat /proc/cmdline 2>/dev/null); do
     esac
 done
 
+# === Drop privileges (respect USER directive from image config) ===
+# /slim/user may contain a username, uid, or uid:gid (OCI image spec).
+# Numeric uids are resolved to a username via /etc/passwd because BusyBox
+# su does not accept numeric arguments.
+SLIM_USER=""
+[ -f /slim/user ] && SLIM_USER=$(cat /slim/user 2>/dev/null)
+
+# Resolve a numeric uid to a username from /etc/passwd.
+slim_resolve_user() {
+    _u="$1"
+    case "$_u" in
+        *[!0-9]*) printf '%s' "$_u" ;;
+        *)
+            _resolved=$(awk -F: -v uid="$_u" '$3==uid{print $1; exit}' /etc/passwd 2>/dev/null)
+            [ -n "$_resolved" ] && printf '%s' "$_resolved" || printf '%s' "$_u"
+            ;;
+    esac
+}
+
+# Exec a command, dropping privileges if SLIM_USER is set.
+# When dropping privileges, run as a child (not exec) so PID 1 stays root
+# and can poweroff after the command exits — non-root cannot call poweroff.
+slim_exec() {
+    _cmd="$1"
+    if [ -z "$SLIM_USER" ]; then
+        exec /bin/sh -c "$_cmd"
+    fi
+    _user="${SLIM_USER%%:*}"
+    _user=$(slim_resolve_user "$_user")
+    su "$_user" -c "$_cmd"
+    poweroff -f
+}
+
 # === Execute the configured command ===
 # If slim.cmd=<base64> is on the kernel cmdline, decode and exec it.
 # Otherwise, exec /slim/exec (generated from the image's CMD/ENTRYPOINT).
@@ -80,9 +113,9 @@ for tok in $(cat /proc/cmdline 2>/dev/null); do
             v=${tok#slim.cmd=}
             decoded=$(b64dec "$v") || echo "base64 decode failed"
             if [ -n "$decoded" ]; then
-                exec /bin/sh -c "$decoded"
+                slim_exec "$decoded"
             fi
             ;;
     esac
 done
-exec /slim/exec
+slim_exec "/slim/exec"