added startup file to handle user configuration

scripts/docker/runit/arm_user_setup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# This script is first to run due to this: https://github.com/phusion/baseimage-docker#running_startup_scripts.
+#
+# It updates the UIG or GID of the included arm user to whatever value the user
+# passes at runtime, if the value set is not the default value of 1000
+#
+# If the container is run again without specifying UID and GID, this script
+# resets the UID and GID of all files in ARM directories to the defaults
+
+DEFAULT_UID=1000
+DEFAULT_GID=1000
+
+if [[ $ARM_UID -ne $DEFAULT_UID ]]; then
+    echo -e "Updating arm user id from $DEFAULT_UID to $ARM_UID..."
+    usermod -u "$ARM_UID" arm
+elif [[ $ARM_UID -eq $DEFAULT_UID ]]; then
+    echo -e "Updating arm group id $ARM_UID to default (1000)..."
+    usermod -u $DEFAULT_UID arm
+fi
+
+if [[ $ARM_GID -ne $DEFAULT_GID ]]; then
+    echo -e "Updating arm group id from $DEFAULT_GID to $ARM_GID..."
+    groupmod -g "$ARM_GID" arm
+elif [[ $ARM_UID -eq $DEFAULT_GID ]]; then
+    echo -e "Updating arm group id $ARM_GID to default (1000)..."
+    groupmod -g $DEFAULT_GID arm
+fi