Update dev.Dockerfile: rebuild buildcache index

containers/jug/dev.Dockerfile

@@ -55,6 +55,7 @@ SHELL ["docker-shell"]
 RUN --mount=type=cache,target=/var/cache/spack-mirror                   \
     export PATH=$PATH:$SPACK_ROOT/bin                                   \
  && spack mirror add docker /var/cache/spack-mirror                     \
+ && spack buildcache update-index -d /var/cache/spack-mirror            \
  && spack mirror list
 
 ## Setup eic-spack buildcache mirrors (FIXME: leaks credentials into layer)