diff --git a/docker_images/mbed-os-env/Dockerfile b/docker_images/mbed-os-env/Dockerfile
index 8e6c08f..942a2b7 100644
--- a/docker_images/mbed-os-env/Dockerfile
+++ b/docker_images/mbed-os-env/Dockerfile
@@ -3,6 +3,11 @@
 FROM ubuntu:20.04
 
 # ------------------------------------------------------------------------------
+# set locale
+ENV LANG C.UTF-8
+ENV LC_ALL C.UTF-8
+
+# ------------------------------------------------------------------------------
 # Arguments
 ARG WORKDIR=/root