[dockerfile] remove _ in xrootd version variable

containers/images/xcache/Dockerfile

 # XCache image
 FROM centos:7
-ARG xrootd-version=4.12.5
+ARG xrootdversion=4.12.5
 
 COPY xrootd-stable-slc7.repo /etc/yum.repos.d/xrootd-stable-slc7.repo
 
 RUN  yum install --nogpg -y epel-release\
-  && yum install --nogpg -y xrootd-server-${xrootd-version}
+  && yum install --nogpg -y xrootd-server-${xrootdversion}
 
 # Have the predefined uid/gid for xrootd to enable easy access to volumes 
 RUN xrootd_uid=$(id -u xrootd)\
@@ -47,7 +47,7 @@ ADD https://indigo-iam.github.io/escape-docs/voms-config/voms-escape.cloud.cnaf.
 RUN chmod 644 /etc/vomses/voms-escape.cloud.cnaf.infn.it.vomses /etc/grid-security/vomsdir/escape/voms-escape.cloud.cnaf.infn.it.lsc
 
 # Install the VO info extractor
-RUN yum install --nogpg -y xrootd-voms-${xrootd-version}
+RUN yum install --nogpg -y xrootd-voms-${xrootdversion}
 
 ENV X509_USER_PROXY=/tmp/proxy-certificate/certificate
 
@@ -55,7 +55,7 @@ RUN mkdir -p /tmp/proxy-certificate\
   && chown xrootd:xrootd /tmp/proxy-certificate
 
 # HTTP xroot client library
-RUN yum install --nogpg -y xrdcl-http-${xrootd-version}\
+RUN yum install --nogpg -y xrdcl-http-${xrootdversion}\
   && rm -rf /etc/xrootd
 
 # problem with xrdcl-http not looking /etc/grid-security