Allow env placeholders for the podman pilot

The podman runtime arguments allows to set environment variable
placeholders starting with '%' and followed by the name of the
environment variable. For example %HOME will be replaced to the
home directory $HOME of the calling user. If the given placeholder
cannot be translated into an existing environment variable it
will stay as placeholder.

doc/podman-pilot.rst

@@ -116,12 +116,19 @@ can be set for the supported container engine:
          - -ti
 
 After reading of the app configuration information the application
-will be called using the configured engine. If no runtime
+will be called using the configured engine. If no podman runtime
 arguments exists, the following defaults will apply:
 
 - The instance will be removed after the call
 - The instance allows for interactive shell sessions
 
+The podman runtime arguments allows to set environment variable
+placeholders starting with '%' and followed by the name of the
+environment variable. For example %HOME will be replaced to the
+home directory $HOME of the calling user. If the given placeholder
+cannot be translated into an existing environment variable it
+will stay as placeholder.
+
 All caller arguments will be passed to the program call inside
 of the instance except for arguments that starts with the '@'
 or '%' sign. Caller arguments of this type are only used for

podman-pilot/src/podman.rs

@@ -182,9 +182,25 @@ pub fn create(
     }
 
     // create the container with configured runtime arguments
-    let has_runtime_args = podman.as_ref().map(|p| !p.is_empty()).unwrap_or_default();
-    app.args(podman.iter().flatten().flat_map(|x| x.splitn(2, ' ')));
+    for arg in podman.iter().flatten().flat_map(|x| x.splitn(2, ' ')) {
+        let mut arg_value = arg.to_string();
+        let re = Regex::new(r".*%([A-Z+]).*").unwrap();
+        for var_name in re.capture_names() {
+            // replace %VAR placeholder(s) into their respective environment
+            // variable if possible. If not possible keep the placeholder
+            // as it is. For example %HOME will be replaced to the
+            // home directory $HOME of the calling user
+            if Some(var_name).is_some() {
+                arg_value = env::var(var_name.unwrap())
+                    .unwrap_or(format!("%{}", var_name.unwrap()));
+            }
+        }
+        app.arg(arg_value);
+    };
 
+    // set default runtime arguments if none configured
+    let has_runtime_args = podman
+        .as_ref().map(|p| !p.is_empty()).unwrap_or_default();
     if !has_runtime_args {
         app.arg("--tty").arg("--interactive");
     }