fix: relative link from epic-main to epic-git.<hash>_main

Instead of

$ ls -al /cvmfs/singularity.opensciencegrid.org/eicweb/jug_xl\:nightly/opt/detector/epic-main
lrwxrwxrwx 1 cvmfs cvmfs 68 Jun  3 14:36 /cvmfs/singularity.opensciencegrid.org/eicweb/jug_xl:nightly/opt/detector/epic-main -> /opt/detector/epic-git.8c40e0ba534f9c3bd7cac3bfcbb46c785b368643_main

this PR will result in

$ ls -al /cvmfs/singularity.opensciencegrid.org/eicweb/jug_xl\:nightly/opt/detector/epic-main
lrwxrwxrwx 1 cvmfs cvmfs 68 Jun  3 14:36 /cvmfs/singularity.opensciencegrid.org/eicweb/jug_xl:nightly/opt/detector/epic-main -> epic-git.8c40e0ba534f9c3bd7cac3bfcbb46c785b368643_main

containers/jug/dev.Dockerfile

@@ -290,8 +290,9 @@ COPY singularity.d /.singularity.d
 ## Fixup /opt/detector/epic-git.fcf90937193c983c0af2acf1251e01f2e2c3a259_main
 RUN <<EOF
 shopt -s nullglob
-for detector in /opt/detector/epic-git.*_* ; do
-  ln -s ${detector} /opt/detector/epic-${detector/*_/}
+cd /opt/detector
+for detector in epic-git.*_* ; do
+  ln -s ${detector} epic-${detector/*_/}
 done
 EOF