Introduce separate ARG for extra packages list

Convert the current PKGS_LIST to ENV as we should not really touch it.
Introduce the EXTRA_PKGS_LIST ARG to allow specifying a file
containing a list of extra packages to install.
The goal is to allow installing packages from different sources
to test them.

Dockerfile

@@ -31,14 +31,18 @@ RUN if [ $(uname -m) = "x86_64" ]; then \
 
 FROM docker.io/centos:centos8
 
-ARG PKGS_LIST=main-packages-list.txt
+ENV PKGS_LIST=main-packages-list.txt
+ARG EXTRA_PKGS_LIST
 
-COPY ${PKGS_LIST} /tmp/main-packages-list.txt
+COPY ${PKGS_LIST} ${EXTRA_PKGS_LIST} /tmp/
 
 RUN dnf install -y python3 python3-requests && \
     curl https://raw.githubusercontent.com/openstack/tripleo-repos/master/tripleo_repos/main.py | python3 - -b master current-tripleo && \
     dnf upgrade -y && \
-    dnf --setopt=install_weak_deps=False install -y $(cat /tmp/main-packages-list.txt) && \
+    dnf --setopt=install_weak_deps=False install -y $(cat /tmp/${PKGS_LIST}) && \
+    if [[ -s /tmp/${EXTRA_PKGS_LIST} ]]; then \
+      dnf --setopt=install_weak_deps=False install -y $(cat /tmp/${EXTRA_PKGS_LIST}) \
+    fi && \
     dnf clean all && \
     rm -rf /var/cache/{yum,dnf}/*