Skip locale-gen for some ubuntu distros

C.UTF-8 is pregenerated anyway, so no need to re-run it.
Also defaults to en_US for centos 7 because C.UTF-8 is not available
there.

Dockerfile.omnibus-gitlab-focal

@@ -6,8 +6,6 @@ RUN apt-get update -q \
         && apt-get clean -yqq \
         && rm -rf /var/lib/apt/lists/*
 
-RUN echo "C.UTF-8 UTF-8" > /etc/locale.gen
-RUN locale-gen C.UTF-8
 ENV LANG C.UTF-8
 ENV LANGUAGE C
 ENV LC_ALL C.UTF-8