Dockerfile: Specify locale settings

Avoid RuntimeError in pipenv by setting LC_ALL and LANG to C.UTF-8.

Dockerfile

@@ -1,7 +1,9 @@
 FROM debian:8
 
 ENV DEBIAN_FRONTEND=noninteractive \
-    DEBCONF_NONINTERACTIVE_SEEN=true
+    DEBCONF_NONINTERACTIVE_SEEN=true \
+    LC_ALL=C.UTF-8 \
+    LANG=C.UTF-8
 
 COPY . /app