cta: use scientificlinux image

cta-build.yml

@@ -13,7 +13,7 @@ before_script:
 cta_srpm:
   stage: build:srpm
   retry: 1
-  image: centos7
+  image: scientificlinux/sl:7
   script:
     - yum install -y gcc-c++ cmake make rpm-build
     - mkdir build_srpm
@@ -32,7 +32,7 @@ cta_rpm:
     - tags
   stage: build:rpm
   retry: 1
-  image: centos7
+  image: scientificlinux/sl:7
   script:
     - cp -f continuousintegration/docker/ctafrontend/cc7/etc/yum.repos.d/*.repo /etc/yum.repos.d/
     - yum install -y gcc-c++ cmake make rpm-build