Add django_ldap_auth

In the ldap.Dockerfile the django_ldap_auth module is installed to
enable authentication againt LDAP servers.

build-branches.sh

@@ -9,6 +9,11 @@ CURL="curl ${CURL_OPTS}"
 
 BRANCHES=$($CURL "${URL_RELEASES}" | jq -r 'map(.name) | .[] | scan("^[^v].+")')
 
+VARIANTS=( "ldap" )
+
 for BRANCH in $BRANCHES; do
   ./build.sh "${BRANCH}" $@
+  for var in "${VARIANTS[@]}" ; do
+    VARIANT=$var ./build.sh "${BRANCH}" $@
+  done
 done