Use remote user from devcontainer CLI and respect shell in passwd (#48230)

Closes #46252

Uses the `remoteUser` property returned from the devcontainer CLI so
that settings are respected. Additionally, checks for a default shell in
`passwd` if `$SHELL` is not set.

Release Notes:

- Fixed remote_user and shell inconsistencies from within dev containers

crates/dev_container/src/devcontainer_api.rs

@@ -19,7 +19,7 @@ use crate::{DevContainerFeature, DevContainerSettings, DevContainerTemplate};
 struct DevContainerUp {
     _outcome: String,
     container_id: String,
-    _remote_user: String,
+    remote_user: String,
     remote_workspace_folder: String,
 }
 
@@ -156,6 +156,7 @@ pub async fn start_dev_container(
         Ok(DevContainerUp {
             container_id,
             remote_workspace_folder,
+            remote_user,
             ..
         }) => {
             let project_name = match devcontainer_read_configuration(
@@ -180,6 +181,7 @@ pub async fn start_dev_container(
                 name: project_name,
                 container_id: container_id,
                 use_podman,
+                remote_user,
             };
 
             Ok((connection, remote_workspace_folder))
@@ -563,7 +565,7 @@ mod tests {
             up.container_id,
             "826abcac45afd412abff083ab30793daff2f3c8ce2c831df728baf39933cb37a"
         );
-        assert_eq!(up._remote_user, "vscode");
+        assert_eq!(up.remote_user, "vscode");
         assert_eq!(up.remote_workspace_folder, "/workspaces/zed");
 
         let json_in_plaintext = r#"[2026-01-22T16:19:08.802Z] @devcontainers/cli 0.80.1. Node.js v22.21.1. darwin 24.6.0 arm64.
@@ -574,7 +576,7 @@ mod tests {
             up.container_id,
             "826abcac45afd412abff083ab30793daff2f3c8ce2c831df728baf39933cb37a"
         );
-        assert_eq!(up._remote_user, "vscode");
+        assert_eq!(up.remote_user, "vscode");
         assert_eq!(up.remote_workspace_folder, "/workspaces/zed");
     }
 }