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-bionic

@@ -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

Dockerfile.omnibus-gitlab-centos7

@@ -6,9 +6,11 @@ RUN yum update -y -q \
         gcc gcc-c++ git make curl zlib-devel openssl-devel \
   && yum clean all
 
-ENV LANG C.UTF-8
-ENV LC_ALL C.UTF-8
-RUN localedef -c -f UTF-8 -i C C.UTF-8
+# it seems C.UTF-8 is not available in centos-7
+# https://bugzilla.redhat.com/show_bug.cgi?id=1361965
+ENV LANG en_US.UTF-8
+ENV LC_ALL en_US.UTF-8
+RUN localedef -c -f UTF-8 -i en_US en_US.UTF-8
 
 ENV RUBY_VERSION 2.6.5
 RUN curl -fsSL "https://cache.ruby-lang.org/pub/ruby/2.6/ruby-${RUBY_VERSION}.tar.gz" \

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

Dockerfile.omnibus-gitlab-xenial

@@ -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