Allow the user that runs the app to be specified via an env var

See gh-16973

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/resources/org/springframework/boot/loader/tools/launch.script

@@ -128,6 +128,26 @@ log_file="$LOG_FOLDER/$LOG_FILENAME"
 # shellcheck disable=SC2012
 [[ $(id -u) == "0" ]] && run_user=$(ls -ld "$jarfile" | awk '{print $3}')
 
+# Force run as informed user (from environment variable)
+if [[ -n "$RUN_AS_USER" ]]; then
+    # checks performed for all actions except 'status' and 'run'
+    if ! [[ "$action" =~ ^(status|run)$ ]]; then
+        # Issue a error if informed user is not valid
+        id -u "$RUN_AS_USER" || {
+            echoRed "Cannot run as '$RUN_AS_USER': no such user"
+            exit 5
+        }
+
+        # Issue a error if we are not root
+        [[ $(id -u) == 0 ]] || {
+            echoRed "root required to run as '$RUN_AS_USER'"
+            exit 6
+        }
+    fi
+
+    run_user="$RUN_AS_USER"
+fi
+
 # Issue a warning if the application will run as root
 [[ $(id -u ${run_user}) == "0" ]] && { echoYellow "Application is running as root (UID 0). This is considered insecure."; }