Converting Docker image to Singularity container

1. run "directly" from DockerHub

This is most common situation, already mentioned.
You can directly run the container. The content of the Docker image will be pulled and cached localy.

$ singularity run docker://godlovedc/lolcow

2. pull/build from DockerHub

With pull the singularity name gets automatically assigned, in this example to "lolcow_latest.sif". Note, this operation does not need sudo.

$ singularity pull docker://godlovedc/lolcow

INFO:    Converting OCI blobs to SIF format
INFO:    Starting build...
...
INFO:    Creating SIF file...

# name gets automatically assigned to "lolcow_latest.sif"

$ singularity build lolcow.sif docker://godlovedc/lolcow

Starting build...
Getting image source signatures
Copying blob 9fb6c798fa41 done  
Copying blob 3b61febd4aef done
...
INFO:    Creating SIF file...
INFO:    Build complete: lolcow.sif

3. Building from docker image on your computer

The examples bellow assume you have permission to run docker.

# Build the docker image
$ docker build -t local/my_container .

# build from local docker repository
$ sudo singularity build my_container.sif docker-daemon://local/my_container

4. Using Docker image saved in a .tar file

# Save the docker container from your computer to a .tar file
$ docker save image_id -o local.tar

# Build from a docker-archive
#$ singularity build local_tar.sif docker-archive://local.tar
$ singularity build local_tar.sif local.tar