context: copy symlinks as symlinks and improve error messages

src/deb/local.rs

@@ -138,10 +138,20 @@ pub async fn build(
         .command("apt-get")
         .envs(env.clone())
         .arg("update")
-        .status()?;
+        .status()
+        .map_err(|e| {
+            format!(
+                "Failed to run 'apt-get update' inside the build context: {}. \
+                 If this is a local build, make sure apt-get is available and \
+                 try executing with sudo.",
+                e
+            )
+        })?;
     if !status.success() {
         return Err(
-            "Could not execute apt-get update. If this is a local build, try executing with sudo."
+            "apt-get update failed inside the build context. \
+             If this is a local build, try executing with sudo, \
+             or re-run with RUST_LOG=debug for more details."
                 .into(),
         );
     }