cta: add build pipeline to build CTA outside of CERN infrastructure

cta-build.yml

+stages:
+  - build:srpm
+  - build:rpm
+
+before_script:
+  - export CTA_BUILD_ID=${CI_PIPELINE_ID}git${CI_COMMIT_SHA:0:8}
+  - echo "Exporting CTA_BUILD_ID=${CTA_BUILD_ID}"
+  - test -n "${CI_COMMIT_TAG}" && export TAG_VERSION=$(echo ${CI_COMMIT_TAG} | sed -e 's/^v//;s/-.*$//')
+  - test -n "${CI_COMMIT_TAG}" && export TAG_RELEASE=$(echo ${CI_COMMIT_TAG} | sed -e 's/^[^-]*-//')
+  - echo "Removing protectbase from all repos (same as Puppet)"
+  - sed -i '/^protect=/d' /etc/yum.repos.d/*
+
+cta_srpm:
+  stage: build:srpm
+  retry: 1
+  image: centos7
+  script:
+    - yum install -y gcc-c++ cmake make rpm-build
+    - mkdir build_srpm
+    - cd build_srpm
+    - cmake -DPackageOnly:Bool=true -DVCS_VERSION=${CTA_BUILD_ID} ..
+    - make cta_srpm
+
+  artifacts:
+    expire_in: 30 days
+    paths:
+    - build_srpm/RPM/SRPMS/
+
+
+cta_rpm:
+  except:
+    - tags
+  stage: build:rpm
+  retry: 1
+  image: centos7
+  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
+    - yum -y install yum-plugin-priorities yum-plugin-versionlock
+    - yum install -y git
+    - git submodule update --init --recursive
+    - cd xrootd-ssi-protobuf-interface && export XROOTD_SSI_PROTOBUF_INTERFACE_VERSION=$(git describe --abbrev=0) && cd ..
+    - cp -f continuousintegration/docker/ctafrontend/cc7/etc/yum/pluginconf.d/versionlock.list /etc/yum/pluginconf.d/
+    - yum-builddep --nogpgcheck -y build_srpm/RPM/SRPMS/*
+    - mkdir build_rpm
+    - cd build_rpm
+    - cmake -DVCS_VERSION=${CTA_BUILD_ID} ..
+    - make cta_rpm
+
+  artifacts:
+    expire_in: 30 days
+    paths:
+    - build_rpm/RPM/RPMS