Set docker image locale to UTF-8.

etc/taskcluster/docker/base.dockerfile

@@ -4,7 +4,10 @@ ENV \
     #
     # Some APT packages like 'tzdata' wait for user input on install by default.
     # https://stackoverflow.com/questions/44331836/apt-get-install-tzdata-noninteractive
-    DEBIAN_FRONTEND=noninteractive
+    DEBIAN_FRONTEND=noninteractive \
+    LANG=C.UTF-8 \
+    LANGUAGE=C.UTF-8 \
+    LC_ALL=C.UTF-8
 
 RUN \
     apt-get update -q && \
@@ -30,5 +33,8 @@ RUN \
         gcc \
         #
         # Installing rustup and sccache (build dockerfile) or fetching build artifacts (run tasks)
-        curl
+        curl \
+        # Setting the default locale
+        locales \
+        locales-all