add default values for ARGs

dev/containers/centos7-rpms-py2

@@ -1,7 +1,7 @@
 FROM centos:7
 
-ARG repo
-ARG branch
+ARG repo=https://pagure.io/pagure.git
+ARG branch=master
 
 ENV REPO=$repo
 ENV BRANCH=$branch

dev/containers/f29-rpms-py3

@@ -1,7 +1,7 @@
 FROM fedora:29
 
-ARG repo
-ARG branch
+ARG repo=https://pagure.io/pagure.git
+ARG branch=master
 
 ENV REPO=$repo
 ENV BRANCH=$branch

dev/containers/fedora-pip-py3

@@ -1,7 +1,7 @@
 FROM fedora:latest
 
-ARG repo
-ARG branch
+ARG repo=https://pagure.io/pagure.git
+ARG branch=master
 
 ENV REPO=$repo
 ENV BRANCH=$branch