replace yum with dnf

docker/centos8/Dockerfile

 FROM centos:8
 
 # add this to avoid locale warnings
-RUN yum -y install glibc-locale-source glibc-langpack-en
+RUN dnf -y install glibc-locale-source glibc-langpack-en
 RUN localedef -i en_US -f UTF-8 en_US.UTF-8
 
 # add utils for repos
 # enable mariadb repo
 RUN wget https://downloads.mariadb.com/MariaDB/mariadb_repo_setup && bash mariadb_repo_setup
 # enable the postgresql repo
-RUN yum -y install https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm
+RUN dnf -y install https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm
 # enable epel repo
 RUN dnf -y install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
 # enable powertools repos
 RUN dnf -y module disable mariadb
 RUN dnf -y module disable postgresql
 
-RUN yum -y install \
-php-cli php-xml \
-MariaDB-server MariaDB-client php-mysqlnd php-json \
+RUN dnf -y install \
+php-cli php-xml php-json \
+MariaDB-server MariaDB-client php-mysqlnd \
 postgresql12 postgresql12-server php-pgsql postgis30_12 \
 sqlite php-sqlite3 \
 git wget