Using docker2singularity to build images from Singularity definition files

[nathanweeks]

Would it be a bad idea to use docker2singularity to build Singularity images from Singularity definition files with singularity build? Lightly modifying the example in the Singularity 3.7 User Guide
, modifying the %files section to copy some file ("test.py") in the current working directory, and using the docker engine from Docker Desktop for Mac 20.10.3:

$ cat singularity.def 
Bootstrap: library
From: ubuntu:18.04
Stage: build

%setup
    touch /file1
    touch ${SINGULARITY_ROOTFS}/file2

%files
    test.py /opt

%environment
    export LISTEN_PORT=12345
    export LC_ALL=C

%post
    apt-get update && apt-get install -y netcat
    NOW=`date`
    echo "export NOW=\"${NOW}\"" >> $SINGULARITY_ENVIRONMENT

%runscript
    echo "Container was created $NOW"
    echo "Arguments received: $*"
    exec echo "$@"

%startscript
    nc -lp $LISTEN_PORT

%test
    grep -q NAME=\"Ubuntu\" /etc/os-release
    if [ $? -eq 0 ]; then
        echo "Container base is Ubuntu as expected."
    else
        echo "Container base is not Ubuntu."
        exit 1
    fi

%labels
    Author d@sylabs.io
    Version v0.0.1

%help
    This is a demo container used to illustrate a def file that uses all
    supported sections.
$ docker run -t --rm --privileged -v /etc/localtime -v $(pwd):/mnt -w /mnt --entrypoint=singularity quay.io/singularity/docker2singularity:v3.7.1 build ubuntu.sif singularity.def 
INFO:    Starting build...
INFO:    Downloading library image
52.1MiB / 52.1MiB [==============================================================================] 100 % 18.3 MiB/s 0s
INFO:    Verifying bootstrap image /root/.singularity/cache/library/sha256.35055a42993db1469734fd488feddc747bdaabc0956b3aabf72c9b1f9385b209
WARNING: integrity: signature not found for object group 1
WARNING: Bootstrap image could not be verified, but build will continue.
INFO:    Running setup scriptlet
+ touch /file1
+ touch /tmp/build-temp-471078291/rootfs/file2
INFO:    Copying test.py to /tmp/build-temp-471078291/rootfs/opt
INFO:    Running post scriptlet
+ apt-get update
Hit:1 http://us.archive.ubuntu.com/ubuntu bionic InRelease
Get:2 http://us.archive.ubuntu.com/ubuntu bionic/main Translation-en [516 kB]
Fetched 516 kB in 1s (693 kB/s)        
Reading package lists... Done
+ apt-get install -y netcat
Reading package lists... Done
Building dependency tree... Done
Note, selecting 'netcat-openbsd' instead of 'netcat'
The following additional packages will be installed:
  libbsd0
The following NEW packages will be installed:
  libbsd0 netcat-openbsd
0 upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 81.0 kB of archives.
After this operation, 270 kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 libbsd0 amd64 0.8.7-1 [41.5 kB]
Get:2 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 netcat-openbsd amd64 1.187-1 [39.6 kB]
Fetched 81.0 kB in 0s (322 kB/s)     
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package libbsd0:amd64.
(Reading database ... 4006 files and directories currently installed.)
Preparing to unpack .../libbsd0_0.8.7-1_amd64.deb ...
Unpacking libbsd0:amd64 (0.8.7-1) ...
Selecting previously unselected package netcat-openbsd.
Preparing to unpack .../netcat-openbsd_1.187-1_amd64.deb ...
Unpacking netcat-openbsd (1.187-1) ...
Setting up libbsd0:amd64 (0.8.7-1) ...
Setting up netcat-openbsd (1.187-1) ...
update-alternatives: using /bin/nc.openbsd to provide /bin/nc (nc) in auto mode
Processing triggers for libc-bin (2.27-3ubuntu1) ...
+ date
+ NOW=Thu Feb 25 21:21:31 UTC 2021
+ echo export NOW="Thu Feb 25 21:21:31 UTC 2021"
INFO:    Adding help info
INFO:    Adding labels
INFO:    Adding environment to container
INFO:    Adding startscript
INFO:    Adding runscript
INFO:    Adding testscript
INFO:    Running testscript
Container base is Ubuntu as expected.
INFO:    Creating SIF file...
INFO:    Build complete: ubuntu.sif
$ docker run -it --rm --privileged -v /etc/localtime -v $(pwd):/mnt -w /mnt --entrypoint=singularity quay.io/singularity/docker2singularity:v3.7.1 shell ubuntu.sif
Singularity> cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
Singularity> ls /opt
test.py

If there aren't too many drawbacks to this approach (I realize this use of %setup wouldn't impact the "real" host), maybe it could be mentioned in the README.md (with an example)? Or would there be a more preferable base image to use?

[vsoch]

That's a cool idea - and I don't see why not! Would you like to create and add a contributed example?

[nathanweeks]

Sure thing! I'll do some additional experimentation over & try to come up with a suitable example within the next 2-3 weeks.