feat: add a container indicator (#3304)

* test: add mock method for absolute files

Signed-off-by: Harald Hoyer <harald@hoyer.xyz>

* feat(module): add a container indicator module

Adds a container type indicator, if inside a container,
detected via the presence of some marker files.

E.g. inside a podman container entered with `toolbox enter`
the prompt changes to the container name and version.

```
starship on  container_rebased [$!] is 📦 v1.0.0 via 🦀 v1.56.1
❯ toolbox enter

starship on  container_rebased [$!] is 📦 v1.0.0 via 🦀 v1.56.1
⬢ [fedora-toolbox:35] ❯
```

Signed-off-by: Harald Hoyer <harald@hoyer.xyz>

src/context.rs

@@ -63,6 +63,10 @@ pub struct Context<'a> {
     #[cfg(test)]
     pub cmd: HashMap<&'a str, Option<CommandOutput>>,
 
+    /// a mock of the root directory
+    #[cfg(test)]
+    pub root_dir: tempfile::TempDir,
+
     #[cfg(feature = "battery")]
     pub battery_info_provider: &'a (dyn crate::modules::BatteryInfoProvider + Send + Sync),
 
@@ -155,6 +159,8 @@ impl<'a> Context<'a> {
             target,
             width,
             #[cfg(test)]
+            root_dir: tempfile::TempDir::new().unwrap(),
+            #[cfg(test)]
             env: HashMap::new(),
             #[cfg(test)]
             cmd: HashMap::new(),