the passed type attribute on destination url will be

• cont.docker OR
  cont.singularity

here is the same format
{ "type": "transfer",
"linkage": "input",
"lfn": "centos-pegasus.simg",
"id": 1,
"src_urls": [

{ "site_label": "local", "url": "file:///Volumes/Work/lfs1/work/pegasus-features/PM-1159/work/vahi/pegasus/blackdiamond/run0018/00/00/centos-pegasus.simg", "checkpoint": "false" }

],
"dest_urls": [

{ "site_label": "local", "url": "symlink://$PWD/centos-pegasus.simg" "type": "cont.singularity" }

] }