app_containers: Detect the user home directory dynamically

...so that we can stop assuming the username inside the container
(although the shortcut generation script still does, but we'll fix that
later, hopefully)

app_containers/.local/bin/run_app_container

@@ -51,14 +51,18 @@ fi
 
 # Default user
 run_as=$UID
-# We assume the user named `user` inside the container always has the same UID as
-# the user with uid $run_as on the host; this is currently not yet configurable
-homedir=/home/user
 if [ "$CONTAINER_RUN_AS_ROOT" = true ]; then
   run_as=0
-  homedir=/root
 fi
 
+homedir=/
+for line in $(sudo cat /var/lib/machines/$CONTAINER_NAME/etc/passwd); do
+  if [ "$(echo "$line" | cut -d: -f3)" == "$run_as" ]; then
+    homedir="$(echo "$line" | cut -d: -f6)"
+    break
+  fi
+done
+
 # Userns-related config
 # Default to identity mapping, which does not provide uid isolation but does for capabilities
 private_users=identity