fix locale settings in our Docker image.

Ubuntu 18.04 does not use default locale UTF-8 which causes problems with special chars
(so far only appeared in one case:
https://gitlab.com/sosy-lab/software/cpachecker/-/commit/f166c511babe34d1bc8ed52b7d28d54850732923#note_1420329522).
The collector for ConfigurationOption uses UTF-8 as default, so we also use UTF-8 as default locale.

docker/ubuntu1804.Dockerfile

@@ -27,6 +27,14 @@ RUN apt-get update \
  && make \
  && make install
 
+# set default locale
+RUN apt-get update \
+ && apt-get install -y \
+        locales locales-all
+ENV LC_ALL en_US.UTF-8
+ENV LANG en_US.UTF-8
+ENV LANGUAGE en_US.UTF-8
+
 # Add the user "developer" with UID:1000, GID:1000, home at /developer.
 # This allows to map the docker-internal user to the local user 1000:1000 outside of the container.
 # This avoids to have new files created with root-rights.