Add username validation

Check that the username is valid before doing any work. Use the default regex from debian's adduser.conf. Will also avoid risk of special characters causing issues.

build.sh

@@ -189,6 +189,11 @@ source "${SCRIPT_DIR}/common"
 # shellcheck source=scripts/dependencies_check
 source "${SCRIPT_DIR}/dependencies_check"
 
+#check username is valid
+if [[ ! "$FIRST_USER_NAME" =~ ^[a-z][-a-z0-9_]*$ ]]; then
+	echo "Invalid FIRST_USER_NAME: $FIRST_USER_NAME"
+	exit 1
+fi
 
 dependencies_check "${BASE_DIR}/depends"