Creating Singularity container from local private docker registry - docker

Question

When creating a Singularity image from a Docker image, how do I correctly reference a Docker image in a local insecure private registry?
Using Singularity 2.2.1, attempted using specfile:
Bootstrap: docker
Registry: localhost:5000
From: tensorflow:latest
%runscript
exec /usr/bin/python "$#"
%post
echo "Post install stuffs!"
With the following results:
Executing Prebootstrap module
VERBOSE [U=0,P=22966] message.c:52:init() : Set messagelevel to: 5
DEBUG [U=0,P=22966] get-section.c:66:main() : Iterating through /proc/mounts
Executing Bootstrap 'docker' module
From: tensorflow:latest
Registry: https://localhost:5000
library/tensorflow:latest
scope=repository:library/tensorflow:pull
URL: https://localhost:5000/v2/library/tensorflow/manifests/latest
Traceback (most recent call last):
File "/usr/lib/x86_64-linux-gnu/singularity/python/cli.py", line 198, in <module>
main()
File "/usr/lib/x86_64-linux-gnu/singularity/python/cli.py", line 146, in main
auth=doauth)
File "/usr/lib/x86_64-linux-gnu/singularity/python/docker/api.py", line 156, in get_manifest
response = api_get(base,headers=token,default_header=True)
File "/usr/lib/x86_64-linux-gnu/singularity/python/utils.py", line 139, in api_get
response = urllib2.urlopen(request)
File "/usr/lib/python2.7/urllib2.py", line 154, in urlopen
return opener.open(url, data, timeout)
File "/usr/lib/python2.7/urllib2.py", line 429, in open
response = self._open(req, data)
File "/usr/lib/python2.7/urllib2.py", line 447, in _open
'_open', req)
File "/usr/lib/python2.7/urllib2.py", line 407, in _call_chain
result = func(*args)
File "/usr/lib/python2.7/urllib2.py", line 1241, in https_open
context=self._context)
File "/usr/lib/python2.7/urllib2.py", line 1198, in do_open
raise URLError(err)
urllib2.URLError: <urlopen error [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:661)>
Executing Postbootstrap module
ERROR: Container does not contain the valid minimum requirement of /bin/sh
DEBUG [U=0,P=22936] fork.c:52:handle_sigchld() : Checking child pids: 22941 22941
Singularity is making an assumption about HTTPS on the private registry.
Thanks,
Piers.

Answer 1

In answer to my own question - it turns out that I need to do two things:
explicitly add http:// to the local registry URL
prefix the Docker image name with "/" so that it doesn't get prepended with "/library/"
Example specfile:
Bootstrap: docker
Registry: http://localhost:5000
From: /tensorflow:latest
%runscript
exec /usr/bin/python "$#"
%post
echo "Post install stuffs!"