Pin xrootd version to 4.12.3 in debian build

Dockerfile_ubuntu_bionic

@@ -46,10 +46,14 @@ RUN apt-get install -y software-properties-common \
       
 
 # Add XRootd repo
+# @todo once the XRootD deb repos are better structured we can point to a
+# particular stable branch. For the moment, we force install the version we want
+# Priority >= 1000 causes a version to be installed even if this constitutes a downgrade of the package
 RUN curl -sL http://storage-ci.web.cern.ch/storage-ci/storageci.key | apt-key add - \
     && apt-get update \
     && echo "deb http://storage-ci.web.cern.ch/storage-ci/debian/xrootd bionic release\n# deb-src http://storage-ci.web.cern.ch/storage-ci/debian/xrootd bionic release" >> /etc/apt/sources.list \
-    && add-apt-repository 'deb http://storage-ci.web.cern.ch/storage-ci/debian/xrootd bionic release'
+    && add-apt-repository 'deb http://storage-ci.web.cern.ch/storage-ci/debian/xrootd bionic release' \
+    && echo -e "Package: xrootd* libxrd* libxrootd*\nPin: version 4.12.3\nPin-Priority: 1000" > /etc/apt/preferences.d/xrootd.pref
 
 # Create local repo from eos artifacts
 ENV EOSREPODIR="/debs/eos/"