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.

src/qemu.rs

@@ -8,7 +8,7 @@
 use anyhow::{Context, Result, bail};
 use base64::Engine;
 use clap::Args;
-use std::{fmt::Write as _, path::PathBuf, process::Command};
+use std::{fmt::Write as _, process::Command};
 
 use crate::{
     forward::PortForward,
@@ -33,10 +33,12 @@ pub struct RunCmd {
     #[clap(long)]
     cmd: Option<String>,
 
-    /// Share a host directory into the VM via 9p. The directory appears at
-    /// the same path inside the VM. Can be repeated for multiple mounts.
+    /// Share a host directory into the VM via 9p. Format:
+    /// `<host-path>` or `<host-path>:<guest-path>`. When the guest path is
+    /// omitted, the host path is used. Non-absolute guest paths are
+    /// relative to the image's WORKINGDIR. Can be repeated.
     #[clap(long)]
-    mount: Vec<PathBuf>,
+    mount: Vec<String>,
 }
 
 pub(crate) fn run(
@@ -88,25 +90,30 @@ pub(crate) fn run(
     }
 
     // Build 9p shares for each --mount.  Tags are short (slim0, slim1, …)
-    // because 9p mount_tag has a ~31-byte limit.  The full destination path
-    // is passed on the kernel cmdline as slim.mount=<tag>:<base64(path)>.
+    // because 9p mount_tag has a ~31-byte limit.  The guest destination path
+    // is passed on the kernel cmdline as slim.mount=<tag>:<base64(guest_path)>.
+    // If no guest path is specified, the host path is used as the guest path.
     let mut virtfs_args: Vec<String> = Vec::new();
-    for (i, host_path) in mount.iter().enumerate() {
-        let canonical = host_path
+    for (i, spec) in mount.iter().enumerate() {
+        let (host_path, guest_path) = match spec.split_once(':') {
+            Some((h, g)) => (h, g),
+            None => (spec.as_str(), spec.as_str()),
+        };
+        let canonical = std::path::Path::new(host_path)
             .canonicalize()
-            .with_context(|| format!("Cannot resolve mount path '{}'", host_path.display()))?;
-        let path_str = canonical
+            .with_context(|| format!("Cannot resolve mount path '{host_path}'"))?;
+        let host_str = canonical
             .to_str()
             .context("Mount path is not valid UTF-8")?;
         let tag = format!("slim{i}");
-        let dest_b64 = base64::engine::general_purpose::STANDARD.encode(path_str.as_bytes());
+        let dest_b64 = base64::engine::general_purpose::STANDARD.encode(guest_path.as_bytes());
 
         // QEMU's QemuOpts splits on commas — escape literal commas in the
         // path as ",," per QEMU convention.
-        let path_escaped = path_str.replace(',', ",,");
+        let host_escaped = host_str.replace(',', ",,");
         virtfs_args.push("-virtfs".into());
         virtfs_args.push(format!(
-            "local,path={path_escaped},mount_tag={tag},security_model=mapped-xattr"
+            "local,path={host_escaped},mount_tag={tag},security_model=mapped-xattr"
         ));
         cmdline.push(format!("slim.mount={tag}:{dest_b64}"));
     }