Install mariadb client for sqlflush

./manage.py dbshell needs the `mysql` command so install it

Dockerfile

@@ -2,7 +2,7 @@ FROM centos/httpd
 
 RUN rpm -Uhv https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-7-11.noarch.rpm && \
     yum -y --setopt=tsflags=nodocs install centos-release-scl && \
-    yum -y --setopt=tsflags=nodocs install rh-python36 gcc mariadb-devel \
+    yum -y --setopt=tsflags=nodocs install rh-python36 gcc mariadb-devel mariadb \
     libxml2-devel libxslt-devel httpd-devel mod_wsgi mod_ssl npm gettext && \
     yum -y update --setopt=tsflags=nodocs && \
     yum clean all